What tires are you running on an 8-8.5" rim on a Duster or Demon? 3.5" bs.

Yup, I agree with @33IMP. With an A-body 8 3/4 and BBP axles you need more like 4.5” of backspace on a 15x8” if you want to maximize tire size. With SS springs you can get away with more because of the high rear ride height, but if you lower the car any even 275’s on those wheels will be into the quarters.