big bolt pattern conversion can you use truck drum brakes

I have a sbp 8-3/4 that I want to convert to bbp. I also have a truck 73 8-3/4 lying around and a truck 75 8.25 lying around, both with 10" drums and 5 on 4.5 bolt pattern. Will the backing plates off of either work with bbp conversion axles? I didn't know if they were the same backing plates as the B body brakes everyone talks about or not, but it would save me some money if I can just use what I already have.