For a 15x10 with a 3” spring relocation and mini tub? With a stock width A-body 8 3/4 with BBP axles?
It’ll depend on the tires you want to run, but the backspace would need to be close to 7.5” with a stock width rear axle.
Without the 3” spring relocation and mini-tub on a Dart there’s no reason to go to 15x10’s at all. Stock spring locations on a Dart mean a 255 is about as wide as you can go, and those fit fine on a 15x8.