bbp axle change what backing plates are needed

Where are you getting the axles from? Everything big bolt pattern OEM is too long to fit an A-body housing and if you get them aftermarket, you need to tell the manufacturer the housing you'll be using as well as the brakes. The bearing location has to be machined correctly for the brakes being used, as well as the register.

I just noticed the earlier post - no, B-body axles are too long to fit and they have a taper preventing them from being shortened & resplined.