Backing plate offset

It all boils down to how bad you want to keep the SBP wheels. The 8 3/4" rear has 5 studs in the housing end and the 7 1/4" has 4. You can either drill the 5th hole in the 7 1/4" backing plates or knock that stud out of the housing end. Assuming the 7 1/4 " rear had 10 inch brakes they would be 10" X 1 7/8" and the supply of replacement parts are drying up. They haven't been produced on a new car since 1972. The price for drums is ridiculous when you can find them and the shoes aren't much better. The B body brakes are 10" X 2 1/2" and since these were produced into the late 80s the part availability is much better (dirt cheap). To use them you will need to redrill your axles to 5 on 4 1/2" bolt circle (if they aren't already) and get a set of 1/2" diameter studs, nuts, and BBP wheels. You pays your money and you takes your chances...