correct backspacing on 2wd 1/2ton stepper???

[72step]

I'm in the process of ordering me some wheels, but who out there know's the correct standard backspacing on the rear of a stepside on a 15x8 inch wheel with a stock diffeential? I currently have 15x10's with 4.5" offset, and the tire is like 3/4" from the bed, and I can't afford to go out anymore because the tire would stick out of the fender, therefore have decided on some 15x8 torque thrust II's Thanks in advance!!!