Trying to figure out the wheel size I need to order.

Dana67Dart said

if you cut it down know that real A-body axles wheel mounting flange to bearing face is different than on a B-E body axle. I believe that affects the backing plate and drums depending on what breaks you go with.

If you don't have too much into it you might want to look for a real A-body rear housing and axles. I have been told that 7 1/4 - 10 x 1 3/4 brake drums and backing plates swap on to a real A-body 8 3/4 housing with real A-body axles.

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Small bolt pattern axles have a different flange offset than big bolt pattern axles. Unless you want to run the small bolt pattern, stay away from OE A-body axles. They’re just a pain, because you have to run the small bolt pattern brakes (10x1.75”). Using the BBP axles you can run 10x2.5” brakes, which are easier and cheaper to find parts for.

I’m not positive, but I’m pretty sure that the only backing plates you can use with the SBP A body 8 3/4 axles are SBP A body 8 3/4 backing plates. BBP backing plates interchange regardless of axle, but the SBP stuff isn’t like that.