Kelsey Hayes disc brakes

The MC simply makes fluid pressure, same pressure in both front & rear systems. The rest of the brake system doesn't depend on whether the MC has a vacuum booster or not. You would probably be happiest with a manual MC w/ bore of 7/8"D or 15/16"D. You can even install a newer aluminum MC (many use 80's Dodge truck), if you buy a 2-to-4 bolt adapter plate ($30 ebay).

The only way to change K-H front disks to BBP might be with an adapter plate, but those are expensive and not liked. Might be better to change to 73+ disks or after-market kit and sell the complete K-H setup. They sell for >$100 here and on ebay. Shipping is the killer.