I went to dual MC in my three 64 & 65 Mopars. In all, I kept the existing distribution block, plugged the rear port, put a 1/4 to 3/16 adapter in the top port and plumbed to existing rear tube w/ 3/16" union. I added an adj prop valve near the MC in case I ever switch to front disks. You can get all the dbl-flare adapters in bubble packs at Autozone.
Many MC's would work, either disk or drum, manual or power. I started w/ an MC from a 99 Breeze w/ ABS, so used on all 3. I used a 2-4 bolt adapter on two, one manual and one w/ original booster. The other used the booster from the Breeze on Dart firewall brackets (in my avatar). For manual, insure you put a new rubber bushing on the end of the brake rod to secure it and get one with the smallest bore you can (mine is 7/8"D).