Your drag advice needed

43” center to center on the spring perches is stock for an A-body rear axle. Spring perches for a 3” relocation would be at ~37” center to center.

Mounting surface to mounting surface for an A-body 8 3/4 is 57 1/8” for SBP axles and brakes, ~57 13/16” for BBP axles and brakes. And you can add about 5/16” to that on each side if you add disk brakes.

15x10’s with a 4.5” backspace and 275’s will be about 3/4” outside the quarters with a BBP 8 3/4 rear, more like 1” with rear disks.

275’s can be tucked in with the stock spring locations and wheel houses on a Duster. With an A-body 8 3/4, BBP axles and disk brakes you would need 15x8’s with about a 4.75” backspace, or 15x10’s with a 5.75” backspace.

With a 1/2” spring offset (perches at 42”) and a 1/2” quarter lip trim I run 18x10’s with 295/35/18’s on the back of my Duster. Wheels are inside the quarters and the car sits fairly low.