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Roof vs headache rack will make a difference. For a 1/4 wave antenna to work properly, you need sufficient ground plane; the center of the roof is where you would want to mount this antenna. If you prefer to mount an antenna on the headache rack, use a 1/2 wave antenna which does not require a ground plane.

The repeater antenna I recommended, the DB404-B has a gain of 5.9dBi, with no beam tilt. That is a great antenna to use if you have it high in the air, and height always out-performs antenna gain. A 10dBi gain antenna at 30 feet will do well in a situation like this, but won't have the range of a 5dBi antenna at twice the height. One must also keep in mind that antenna gain comes as a compromise. To make gain, the antenna must have a narrow beam-width... as in the antenna system is designed to pull the energy up off the ground, and down out of the sky, focusing it into the horizon. If you have a high-gain antenna on a tall tower, it will have great distance range, but will also provide very poor results up close to the tower site. The signal will simply over-shoot the target radio. Real world case: I take care of a UHF Ham repeater in Campbell Nebraska on 444.475/449.475. It is using a DB420, which has 11.3dBi gain, and it is mounted at about 290 feet on a commercial tower. Other repeaters link into this site from well over 40 miles away. I live 7 miles away from this site, and can use a 2-Watt handheld from my back yard just fine. If I travel closer to the machine, I drop in and out, and can no longer hold the machine when I'm 1-3 miles away, due to no ground coverage, and signal overshoot. I'm inside the shadow ring of the antenna system. Summary: Use a higher elevation, and a not-quite-so-high gain antenna for a project like this ranch. The lower gain antenna will have much better ground coverage near the tower site, without shadow rings around the site, and the height will make up the distance covered.