My B-Body had the same 4 piston setup. I updated the spindles, rotors, calipers, and kept the original proportioning valve, master cylinder and power brake booster. My brakes stop my 6 pack just fine. I didn't think I'd have adequate vacuum with my high compression motor, but it seems real good. I think I'm using 10.78 inch rotors and premium semi-metallic pads, and with just a single piston Cuda/Challenger caliper. Hope this helps...
KK