Well. I believe my T-680 is a bit different than your pick-up. Just trying to get a feel for what most cb'rs are using. What they find to be the best choice. And yes, I know the application is going to be different from truck to truck.
I would measure from the ground up to the mounting location and subtract that measurement from 13 foot 6 inches and whatever you come up with as far as a measurement that's what I would be looking for in the length of an antenna. Preferably center load over a bottom load . .
I find that my Sirio 5000 out performs my Wilson 5000 by several more miles in a side by side test when I was parked. I could hear and be heard by stations with the Sirio that the Wilson couldn't make the trip by 10 or more miles. And my Wilson talked very good. But I've found something much better.