What is your tolerance for large antennas?
What will work best is a 102" 1/4 wave whip. But on a truck like that, it looks pretty odd. An 8 1/2 foot long whip antenna banging around gets old after a while. In the old days, people use to tie them down, but that will impact performance.
You can use shorter/loaded antennas. Trouble with mounting it there is you have a lopsided ground plane. Ideal location is center of the roof, but looks like that's not what you want.
Personally, after 30 years of messing around with this sort of stuff, I gave up on consumer grade antennas. When I use a CB (rarely), I use a Larsen NMO-27. It's a base loaded whip. The one I use is nearing 30 years old and still works like new. It's a base coil about 3" tall and a 47" or so long thin whip antenna. Won't stand out much. The whip is very flexible, so it'll take off road type abuse well.
Firestick seems to be popular amongst some CB'ers, but I never liked the thick fiberglass antennas. Those that have them seem to like them.
