7 1/4 rear end axles

Why not take the guts out of your SBP rear end and transfer them to the BBP rear end?
It seems like all you want is the BBP rear end with the bigger brakes etc. so that you have the same bolt pattern front and rear.
The axles themselves are not interchangeable, unfortunately.
The problem is, that the cases might not be machined identically between the housings so that the pattern on the ring and pinion are the same.
Before you disassemble your good small bolt rear end, check the gear pattern and the back lash, and if it isn't close in the BBP rear housing, then shim it to be the same.
There should be enough parts between the two to make it happen.
Good luck, and let us know how you make out.