8.25 Vs 8.75

As Loco said, the BBP backing plates won't work with the SBP axles, different offsets. Once narrowed to A body dimensions, your SBP axles will fit the 8.75 housing without issue; you just have to find the correct pre-'73 A body brake parts. 8.75 rear will also require shortening the driveshaft if you are replacing a 7.25, as well as addressing possible u-joint differences.