It isn't easy to find an original 4-bolt booster, but if you can that would be easiest, though perhaps not cheapest. Harder still, is to find the "stand-off brackets" for an A-body. Once you have those, you can mount many booster/MC's. I put one from a 95-99 Breeze on my 65 Dart (see Avatar), with slight rat-tail filing of the mounting holes. That was easy and cheapest for me.
Easiest (if you include seaching time), would be to buy the $170 brackets-booster-MC on ebay. It uses a GM booster, but connects to your pedal. That gives you a dual reservoir MC. If you insist on using your single MC, try a 2-4 bolt adapter plate ($30 ebay), but those are designed to go the other way (mount newer 2-bolt MC to old 4-bolt booster or firewall).
If you do go to a dual system (like most here), many (like me) re-use the distribution block, by plugging the rear tube's port and using a 1/4 to 3/16" flare adapter (Autozone) in the top port. Get the correct adapters. Don't screw a 3/16" tube directly into that top port (even though you can find nuts that fit). BTW, the dual system was a fed mandate. For those that insist on a dual system, why stop there? Go to the current "X system" (RF-LR, LF-RR), but it is quite a plumbing job.
Don't believe the claims that 9" brakes can't stop your car safely. If they can lock up the tires, no brakes can do more. Their deficit is that they don't cool off fast, so repeated braking can quickly overheat them and cause fade. That is why the fed's mandated front disks. Too many Floridians were riding the brakes in the NC mountains and running over the side. People who know how to drive (rare today) don't do that. Drum brakes are also touchier to any gunk on the shoes, giving unequal braking. But, that isn't a problem if well-maintained. Many here change from neglected drums to new disks and notice a great improvement, which shouldn't amaze anyone.
