You best bet is to get under there with a measuring tape and straight edge with the truck at ride height and the box on to figure out your backspace to get the look you want. I always want to get the wheel as far out as possible to make the fenderwell look full, but the look you're after will affect your backspace. Also have a look at what wheels you want and what the standard backspace is on wheels of that size and style.

I have 15x10" wheels with 3-3/4" backspacing on the back of my '46, but that's also with stock fenders and an S-10 rear end, and the box is tubbed.

Last edited by 46gmcpu; 04/17/2013 12:50 PM.
1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket