Need 15" x 10" Rally Rim & Tire Sizing Help

[1986swb]

Sorry if this is a redundant thread but after searching the forum, I could not find the answers to my questions applicable to my 1986 GMC Fleetside SWB, with stock ride height and using 15" x 10" rear rallies:

What is the most effective (without scraping or rubbing inner and outer fenders) back spacing rim size AND maximum tire size combination for my 86 SWB?

1. Back space = 4.5" ? Max. tire size?
2. Back space = 5.0" ? Max. tire size?

My SWB currently sports the 15" x 8" rallies on 255-70s, with 4.5" back spacing. See pic.

Thanks guys. Your recommendations and expertise are greatly appreciated.