I ran 295/50s on 15x8 rims with 4.75 back space on the stock location suspension with no interference. It was very close to the rear leaf segment, but it didn't rub.
I am now running a 305/45 on 15x10 with 5" backspace, springs moved in 1" per side, and a '67 B body housing. I have plenty of room all around.
If you only offset the leafs 1", you could run a 5 to 5.5" backspace rim.