B body rear brakes onto A body axle

Wow. Ok,

1. Small bolt pattern axles can only be used with small bolt pattern brakes. That's it. Even if you re-drill the stock axles to BBP, you still have to use SBP backing plates and SBP drums and re-drill them as well. The axle flange offset is unique to the A-body SBP axles, nothing else has that offset.

2. Big bolt pattern brakes can only be used with big bolt pattern axles. That means you can not use big bolt pattern rear drums and backing plates (10x2.5, 11x2, 11x2.5, or 11x3) with ANY stock A-body 8 3/4 axle, as they were all SBP from the factory.

So, if you have 10x2.5" drum brakes and want to use them, you need to buy aftermarket axles and advise the company you plan on using BBP brakes so the axle flange offset is correct for the brakes you intend to use. Call Cass at Dr. Diff, he'll set you straight.

Otherwise, you need to use A-body 10x1.75" brakes (10x2.25" drums are for the front). If you re-drilled the stock axles to BBP you still have to use the SBP brake hardware, you'll just have to re-drill the SBP drums to BBP too. There are no drums you can order that have the 5x4.5" bolt pattern and use SBP axle flange offset.

A-body SBP axles use a different axle flange offset than BBP axles. They can not be interchanged. The BBP axles all used the same flange offset, so you can use BBP brakes off of anything. And the backing plates and drums are NOT interchangeable between the SBP brakes and the BBP brakes. The difference isn't just in the drums, it's also in the backing plates. So, 10x1.75" backing plates can not be used with 10x2.5" drums and shoes or vice versa, the depth of the mounting flange on the backing plate is different. Either re-drill your SBP hardware to match your re-drilled SBP axles, or buy BBP axles to fit your A-body rear axle housing and use the BBP brakes.