Yeah I really don’t think I’d have an A-body 8 3/4 narrowed to run a 1/2” spring offset in stock tubs.
On a stock Dart with an 8 3/4 and BBP axles the stock spring location is about 6” from the wheel mounting surface back to the spring. So figure with a 1/2” offset it will be about 6.5” with a stock width rear.
With a 275/60/15 you’re gonna want close to a half inch of spring clearance, and you’ll have a bit more than 1” of tire overhanging the lip of the rim. So you’re gonna want a 15x8 with a backspace of like 4.75” to 5”. That’s totally a backspace number you can get even with off the shelf 15x8’s.
Obviously you should measure your own car, since it’s gonna be tight and the body tolerances were pretty loose. But I really don’t see a need to narrow the rear axle. If you were cutting down a wider rear axle it would make more sense, or doing a mini-tub and running 15x10’s which come in a fairly limited range of backspace etc. In your case though a 15x8” with a 5” backspace should be pretty darn close, and it’s not like shortening the axle is gonna give you more real estate, it just changes the backspace needed.
