All BBP backing plates are interchangeable, body style doesn't make a difference. The only reason there are different A-body backing plates is because of the SBP axles. With BBP axles, the B-body plates will work fine.
The emergency brake cables are not axle specific, you can use the 8.25" cables on an 8 3/4. All 8.25" backing plates would be for BBP axles and would have the BBP axle flange offset, so as long as you're using them with BBP axles they will work with an 8 3/4 housing.
