I have a 69 Javelin SST with an 85 Camero brake booster assembly in it.
I was rebuilding my Javelin when my Brother was rebuilding his Camero. We striped a parts car for him at my house. I was amazed at how similar the booster and mouting bracket were to my car.
Since I needed to replace mine, I took a few measurements.
The only difference is that the AMC system bolts to 4 studs that are pressed into a bracket behind the firewall. The Camero system has 4 bolts pressed into the bracket. Since the studs were tack welded to the GM bracket, I removed the studs in the firewall and the whole system bolted right in.
I used the GM propotioning Valve. It works great for me.
The parts are cheap and readily avalible.

Jim