Correct. The only brakes that will fit re-drilled, stock A body axles are the original 10x1.75" brakes. For which you'll have to get the drums re-drilled to BBP, a process you'll have to repeat every time you need new drums.
The axle length and offset are different for the SBP A body axles.
I assumed the 8 1/4 was BBP already and didn't need to be re-drilled.