Need 5th Wheel Advice - Which Cab & Box on GM HD2500

I've had extended cab and crew cab trucks but always 6-1/2 ft bed because being garagable is important to me. I don't have a slider hitch and never had a problem. 2500 or 3500 your choice and while price diff is minimal, the 3500 will have a few less standard features.

My one new point, pay attention to fuel tank capacity. I don't know about 2016/17 but in 2013 the short bed ext. cab had a 26 gal tank, all the other configurations have the larger tank.