The problem is that you’ve got an 8.25 or 7.25” rear axle. They’re not the same width as an A-body 8 3/4. In fact they’re about a full inch narrower. That’s a 1/2” per side that you don’t have, and that’s why those 275’s rub.
A 15x8 with 4.5” of backspace works for an 8 3/4 with BBP axles, not a 7.25 or 8.25.
