9" to 10" rear brake converion

Which axle are we talking about? When you said you had 9" brakes, I automatically assumed you had a 7 1/4" rear end. To the best of my knowledge, ALL A-body 8 3/4" axles had 10" brakes. To change these, use the appropriate backing plate/drums, redrill or replace the shafts and Bob's your uncle.

If it is a 7 1/4" axle with 9" drums it is not so easy. The 9" brakes themselves were narrower, and therefore the housing was wider. Best bet here would be to swap the whole rear end for a later big bolt pattern unit, all of which should have 10" brakes.