Rim and Wheel size help?
 

I have a 1972 C/10 Step-side short bed. I want to run a 15" rim with the tallest and widest tire possible without chopping the frame. I want to tub the bed and at worst, roll the fender edges. I will be changing between slicks and drag radials for the street. I am sure someone has "been there, done that". What back spacing and tallest tire?
Any in-sight would be appreciated!