Question after converting a 7-1/4 to 8-3/4

Yes, the 7 1/4 SBP backing plates won't work on an SBP 8 3/4. The BBP 7 1/4 ones will fit if you have the BBP axles for the 8 3/4. The SBP 7 1/4 brakes will swap over to the SBP 8 3/4 if you get the correct backing plates. Driveshafts have to be shortened going from a 7 1/4 to 8 3/4. You'll need the correct, bigger U-bolts and shock plates too.