Jones

Forget all of that bulkhead connector stuff.  That is a problem waiting to happen.  If the cab ever has to come off, just replace the whole mount if needed. That would be cheaper than all of the added connectors for that bulkhead setup, and those connectors ARE lossy.
 
Here's another better idea... just drill a hole in the top center of the cab roof, and mount the antenna there.  If the cab ever needs to come off, the antenna comes with it.  You will also get better results with the antenna on the roof than you would with it on the rack due to a more evenly distributed ground plane around the base of the antenna.

Forget all of that bulkhead connector stuff.  That is a problem waiting to happen.  If the cab ever has to come off, just replace the whole mount if needed. That would be cheaper than all of the added connectors for that bulkhead setup, and those connectors ARE lossy.
 
Here's another better idea... just drill a hole in the top center of the cab roof, and mount the antenna there.  If the cab ever needs to come off, the antenna comes with it.  You will also get better results with the antenna on the roof than you would with it on the rack due to a more evenly distributed ground plane around the base of the antenna.

Height is everything.  The further up in the air you can go, the better, and longer range you will have.  On the roof would be ideal, if you can hide it somehow.  ...perhaps connect it to a vent pipe on the roof, and paint it some flat, light gray color so it doesn't show up unless you are looking for it.
 
The attic would be OK, as long as you don't have those metal-backed shingles on your roof.    Those will kill the signal.

"...Just wasn't sure if using smaller diameter cable would have an effect on the radio performance per some of the discussion I read about."
 
The small cable to stay away from is RG-174u and its equivalents.  That is the small stuff you find as the stock cable on the Midland Micro-Mobile antennas.
Most NMO mounts come with preinstalled RG-58u, which is slightly smaller in diameter than a #2 pencil.  RG-174u is about the diameter of the ink tube inside of a Bic pen.

Yes. Put very simply, no antenna has any gain as they are passive devices, they simply radiate. True gain requires amplification. What is called gain in an antenna is really apparent gain. That is, the the strength of the received signal, either at or from the remote radio is compared to the signal received using an hypothetical lossless antenna that radiates or receives signals equal from all directions - above, below and from the sides.  If an antenna's design favors signals transmitted or received from some particular direction, or directions, then at the same time it is reducing the signal transmitted or received from some other direction.  The result of changing the radiation/reception pattern results in signals from/to some direction(s) being stronger than from others. Thus the apparent gain.   Your doughnut radiation pattern favors signals from/to the sides and reduces signals sent/received vertical.  
 
It is a little bit like adding a lens to a lamp. A bare bulb (theoretically) radiates light in all directions. But, by placing a reflector behind the bulb and a lens in from of the bulb to direct the light, you will have much more brightness in the direction the lens faces, and much less light behind the reflector. But, the actual light output from the bulb remains unchanged.  In fact, Yagi design directional antennas use elements with those exact names: directors and reflectors.

That stubby antenna is more than likely a 5/8 or 3/4 wave normal mode helical antenna.  It would have not as much gain as a true 5/8 wave, but due to a low-angle "flattened doughnut" radiation pattern, it would have slightly more gain than a 1/4 wave antenna.  ... theoretically.

A UHF Phantom antenna is NOT 3-5 dB gain. At best, it's a zero gain.  You'll see them advertised as a 2.5 gain dBi (Gain over an isotropic) - which is Zero dB gain. Some people decide that 2.5 dBi rounds up to 3 dBi for advertising purposes, and since most people don't know or care to figure out what the difference is - it means that dBi ratings sell more antennas.
 
If you want a true Gain antenna in UHF, you're going to need something taller than 1/4 wavelength.
 
That said, the Phantom antennas are decent enough for what they are, but don't expect any miracles.

You would lose, not gain signal by moving the antenna down 15 inches to that tool box. If you can't bring yourself to drill a hole for a proper antenna, then get a better mag mount than the stock Midland (which is junk) with a better coaxial cable, (not that ultra-thin 174 junk that Midland uses) and leave it on the roof.  THAT is where you will get best results on UHF.  Even just a better 6" quarter-wave with a good cable and lager, low capacitance mount will beat the Midland quarter wave.

Yes, it grounded through the antenna, and/or the mounting bracket of the radio. (Kenwood 880s are not floating case. the metal case and bracket IS negative ground.)  That's another reason NOT to fuse the negative power lead of a mobile radio. If that negative fuse blows, you might not ever know until you burn up the shield on your coaxial cable by pulling 20 Amps of DC across it.  Also, I don't know where you learned the bit about "Connecting positive first as always" ...but that only applies to replacing the battery.  It is totally backwards with regard to hooking up accessories in a negative ground automotive situation.  After everything is mounted and grounded, then and only then should you hook up the positive "Hot" wire,, and install its fuse LAST.

Yes, it grounded through the antenna, and/or the mounting bracket of the radio. (Kenwood 880s are not floating case. the metal case and bracket IS negative ground.)  That's another reason NOT to fuse the negative power lead of a mobile radio. If that negative fuse blows, you might not ever know until you burn up the shield on your coaxial cable by pulling 20 Amps of DC across it.  Also, I don't know where you learned the bit about "Connecting positive first as always" ...but that only applies to replacing the battery.  It is totally backwards with regard to hooking up accessories in a negative ground automotive situation.  After everything is mounted and grounded, then and only then should you hook up the positive "Hot" wire,, and install its fuse LAST.

The only few times I have had that kind of a voltage loss, the problem was solved by replacing, or in one case, upgrading the stock battery cables.
 
If you are grounding an amplifier system in the trunk of a vehicle, it is sometimes necessary to bond the rear ground bus to the front of vehicle ground bus with a copper strap or #8 wire.  Some cars are not as conductive as others - too much aluminum and plastic these days.
 
There have also been a few instances where I have had to run an additional ground strap from the body ground to the case of the alternator to get rid of "whine" because the factory engine block ground isn't low-impedance enough. ...again, particularly when there is a lot of aluminum and plastic involved on the engine.
 
I guess every install is different, but just hooking accessories straight to the battery, while solving some problems, will create other potential problems....corrosion being #1.

I'm of the school of professional installers (MECP) who never fuse the negative side of electronics in a modern negative-ground vehicle, but then I also NEVER hook anything directly to the battery of any vehicle made after 1995 or so.  That really old-school thought from the 60s and 70s needs to be re-educated.
 
If you think you absolutely MUST hook the radio "straight to the battery", then I guess you should fuse both leads.  If you want to do it right, the negative lead should be hooked directly to the body electronics common, or "Star" ground point of the vehicle, (the primary ground for body accessories, isolated from starter) and the positive should be hooked to the vehicle manufacturer's designated accessory power takeoff point with the proper sized fuse as close the the power takeoff point as possible.  (Power takeoff point is usually located within 2 feet of the battery, fed with a LARGE red wire, and is normally a plastic-capped block with a large bolt that has everything else hooked to it.)
 
If you install the accessories like the car manufacturer intends you to do, and you never hook anything straight to the battery, then you won't need to worry about the starter back-feeding through your antenna and blowing up your radio.
 
Hooking a radio straight to the battery in any modern car is foolish, and can cause noise pickup from the car's other electronics.

You should be able to get full-scale copy at 3.5 miles on less than 2 watts.  If you can't do it at 40 Watts, then lose those junk Midland antennas, and get a couple of decent UHF omni antennas mounted on the roofline.  Also run a heavy, large gauge coax cable designed for UHF...such as LMR-400.  If you have those Midlands with that thin RG-174 type coax, then that is exactly why you can't get 3 miles.  If you need to extend the coax, note that the CB coax extenders you buy at the truck stop will not work at UHF.

You should be able to get full-scale copy at 3.5 miles on less than 2 watts.  If you can't do it at 40 Watts, then lose those junk Midland antennas, and get a couple of decent UHF omni antennas mounted on the roofline.  Also run a heavy, large gauge coax cable designed for UHF...such as LMR-400.  If you have those Midlands with that thin RG-174 type coax, then that is exactly why you can't get 3 miles.  If you need to extend the coax, note that the CB coax extenders you buy at the truck stop will not work at UHF.

Omni's fall into a similar issue.  While there are some dual-band 2m/70cm vertical antennas that cover from 440-470 MHz, the gain is very low.  As gain increases, the bandwidth narrows and so does the takeoff angle, impacting sensitivity between stations with elevation difference.
 
So, if you have the expendable income, you can test it.  However, don't be surprised if the lower gain of a broad band antenna doesn't help much.
 
I have to agree with RCM on getting two service-dedicated Larsen welded Yagis.  That's going to be your best bet, relating to both performance and price.

Gawd I hate politicians!!!!
They tell me what radios i can use.
The tell me what guns i can own (i know, not for this forum)
But damnit,
I dont hurt people
I dont tell people what to think.
Why cant Politicans just leave me alone!,
 
 
 
Ok, thxank you for letting me vent!
 
If thes dialog is innapropiate for this forum. PLEASE DELETE IT.

No critique needed.  This looks like a great solution for your particular installation.  ...and even fairly in-expensive.
 
As for the radials being longer than the calculated length, keep in mind that an NMO feed point is elevated up from the plane level, thus the plane surface area will need to be slightly longer.  Follow Laird's directions.

I'm not trying to endorse any specific battery, and I am not affiliated with the company in any way, but I have had very good luck in the past with Rayovac brand alkaline batteries.  I can usually find them at MUCH cheaper prices than the Energizer or Duracell brands, and they work every bit as well.
 
This statement is based on my experience and opinion. As always, YMMV.

Everyone in your immediate family would all use the same call sign. You all have to ID with the call sign every 15 minutes of conversation and at the end of the conversation, regardless of the duration.
 
You can add additional identifiers if you would like. An example would be, everyone in my family would use ABCD123, but I may ID my station as ABCD123-ALPHA. My wife would ID as ABCD123-OMEGA. My son could be ABCD123-BETA.
 
Again, the add-on ID is optional... but legal. For what it's worth, no one in my family uses add-ons. We all just use ABCD123 and call each other by name.

Yep, you all use the same call-sign, and it is a 15 minute interval, unlike the 10 minute interval in ham radio.  As Marc said, identifiers are up to you, not regulated or required.  In my family, we use unit numbers. I'm unit 1, (ex) wife (was) unit 2, sons are units 3,4, and 5, cousin Mike is unit 6. Grandpa goes by a handle: "The Old Timer". Still same call.

Kenwood has it's own quick connect on the radio, however many of us in GMRS, and almost everyone in ham is using the 15/30/45 Powerpole connectors from Anderson.
 
https://en.wikipedia.org/wiki/Anderson_Powerpole

Many of us here are hams.  We have a ham radio section on this forum: https://forums.mygmrs.com/forum/10-amateur-radio-ham/
 
We will be happy to answer your questions.
 
Some here will be happy to give you advice, even if you DON'T want it. hahaha
 
Seriously, we are here to help.

Just another clarification for those not already in-the-know... TA stands for "Talk Around", which talks "around" the repeater, in other words, simplex on the repeater output frequency.  If you set up channels 15-22 all as +5MHz repeater channels, then pushing the TA button makes them all simplex channels for talking radio-to-radio directly.
 
Set up the 16th channel for whatever repeater you will normally be using that might have a non-standard tone set.

I have to agree with you on the trunking radios.  I have two TK-885 units that are the trunking version of the 880.  They are a nasty radio to program into conventional mode,  They do work, but I can't get them to scan.  Also, the conventional mode is limited to 36 channels on the 885.  I'm not sure on the 863.  I have a couple of 862 units, which are conventional.

6 inches or more is a near perfect ground plane for UHF, so your proposed 8 inch plate would be great just as it is.  You should get good results from that.
Make sure your headache rack is grounded to the truck's frame and body to avoid static. as in, if it is mounted on rubber bushings, run ground straps or wires as needed.