Wheel to spring clearance

Sure, but what rear springs were you running?

The picture in your photo garage says it all- the section width of those tires is even with the outside lip of the quarters. The only reason they don't rub is the massive rear ride height. If there wasn't a 4"+ gap between the top of the tire and the quarters, they'd rub. The stock quarter lip is 1" wide, those tires will never clear that if they ever ended up at the same altitude as the quarter lip.

With an A-body 8 3/4, BBP axles, stock springs and 275's on a 15x8 you want 4.5" of backspace. The tires will fit inside the quarters, and you can run at stock ride height or even a little lower. Otherwise you have to run it 70's street freak style with the rear end of the car jacked way up in the air. Which is fine if that's what you want, but 275's will actually fit inside the stock wheel houses with the proper backspace

View attachment 1715276843

That's an optical illusion in the photo. The tires did fit inside the lip. It took 600 lbs in the trunk to find out for sure but they did fit by 1/2 an inch. Those are SS springs by the way.

Jack