Problem with box mounts by the cab is you get alot of SWR because your signal gets re-absorbed into the cab (part of your ground plane in essence)

I have a 9 foot whip on my half-ton that's mounted on the drivers bed rail, but halfway back (directly over the tire). That matches great. My buddy has a whip on the passenger bed rail of his ranger all the way to the back, it also matches well. My dodge has a Coil load (think trucker's antenna) on it, mounted to the top rail of the headache rack on my flatbed. Matches pretty well too.... (Though not as good as the 9 foot whip on the chevy. lol)