Well, attic and indoor certainly helps with SO239 and crimped cables rusting outdoors... but LMR400 in general is just too hyped. Honestly I would ditch it and get Heliax. You can find surplus heliax quite affordable.... but if you must have LMR400 at least use silver plated solderless N connectors like these:
https://www.ebay.com...872.m2749.l2649. I have a small 4 feet patch between the top arrester and the antenna, the N side is using such connector... and the other side is using a 100% silver plated UHF connector, sold from the same guy. (since antenna is UHF...) next antenna will be N connector... and hopefully tri-metal too... we'll see.
Again, I strongly advice to ditch the SO239 as early as possible in your hobby endeavor. Otherwise you'll have to redo everything at a cost later...
As for grounding, sure, the metal mast I am using is mounted on the roof with no grounded metal nearby either so I ran a solid 10AWG copper wire to get it grounded directly to the rod at the electrical box. Ideally you'd want to ground yours to the outside ground rod too, and use the same cable for grounding all stuff... but any metal pipe that is grounded should work, just make sure its grounded, tho. I would still ground everything to the same ground wire if possible.
I've found out that the more I improved the grounding on the mast/antenna/radio the more it made cable lengths less important for SWR. When I had the antenna 100% ungrounded, then using different cable lengths gave vastly different SWR readings at the radio end, but once I started grounding it, the SWR differences between lengths became much smaller.
The explanation I've read for this is that verticals tend to have issues with feed line decoupling (I am using a TRAM 1450 vertical for GMRS) whereas higher quality antennas like the DB404 folded dipoles tend to not have this kind of issues due to how folded dipoles work.
Arresters, I think they are great for grounding the feedline, and I suggest using two, even for an attic setup like yours. One placed right under the antenna and another somewhere closer to the radio. I've had very good results with these Times Microwave LP-BTR-NFF, which are fairly inexpensive and use tri-metal instead of chrome plated connectors.
Lastly, I also recommend coiling two or three times the patch cable going from the top arrester to the antenna with around 8 inches diameter in each turn. That in conjunction with a sold ground should improve noise threshold quite a bit. Having the arresters at each end of the mast also made the installation really manageable, since now I can bring the antenna(s) down without having to remove or deal with the 20 feet Heliax anaconda feedline...
Thanks for the tips. If I go over the 100 foot mark by moving the antenna out of the attic and onto the roof I will definately look into the Heliax. For now I am going to stay with my 55 foot run into the attic. I originally planned on a 90 foot run using a 15 foot pipe next to the house but went with an attic mount solution to avoid outdoor elements and allow me to shrink my cable length needs. Was a good thing too. I had enough extra to cable to allow a couple of "whoopsies" with my initial attempts at using crimp connectors on the LMR400.
Along the line of grounding: Since I am mounted on an attic rafter, I don't have any metal contacting the antenna at all. I don't suppose I could just run a ground line to the plumbing vent in the attic? Run a grounding line down to the basement and ground to the well pipe coming into the building?
Thanks again for the advice!