There were only 3 brake systems on 67-72 A-Body's. 10" drums all around, 9" drums all around, or 4-piston discs in the front and 10" drums in the rear. If you are talking about later A-Body's you could still not mix 9" and 10" brakes.

The funny thing about science is that if you change one miniscule parameter you change the entire outcome to the way you want it.

JB Rhinehart, Realist

A-Body's RULE!