Truck for Eagle Cap 1165?

Talked w/ local dealer today about getting a F450 w/ box and non-detuned motor and dropping box for custom flatbed. He thought box was only few hundred bucks difference. He also mentioned how hard it is to come up w/ real payload numbers off of factory provided numbers. Also said only get 4.33 gears and "6.7 a good motor". He can not order 550's.
My center of gravity is 8" behind my axle on a 8' foot box. I've been wondering what a longer wheelbase does for turning? Somewhere on a Dodge page I found some turning radius numbers but lost them.