I have 275/60/15 MT radials on 7 wide Weld prostars with 4 and 3/4 BS. Got 3/4 in spacing on both body and spring side with no rubbing. It's as tight as I would want to go. No issues so far tho. Changing rims is difficult tho. Gotta drop the rear susp. down fully to change.
These cars are always an issue to stuff large "meats" back there. A well planned spring relocation is probably the best solution.