What size is your exhaust and how is it routed now? Are you willing to have it exit in front of the rear wheels?
I've been able to run 15" x 8" with 4.5" backspacing and 255/60-15's with 2.5" exhaust that exits in the stock location, but I think I remember the exhaust guy putting some extra effort into the pipes between the tires and frame.
Changing to 4" backspacing would help with exhaust clearance a lot.
You can possibly squeeze some 275/50-15's back there too. I have done that, but if I remember correctly that was after my exhaust had been changed to exit before the wheels.