The old cast-iron ones always leak from the stamped-steel covers. That is why every used old Mopar in the junkyard has a rusty firewall around the MC, since glycol fluid is a great paint remover. That also lets moisture in to corrode the innards of the brake system. Many here use a 1980's Dodge truck MC, which is what the Dr Diff kit linked uses. Except Dr. Diff also sells those custom-made with a smaller 15/16" bore. That is desirable if manual brakes and you switch to front disks.
I use a ~1999 Plymouth Breeze (Cirrus, Stratus) MC in all 3 of my 1960's Mopars. I bought a 4 to 2 bolt adapter plate on ebay ($30, same as Dr. Diff's). One car (64 Valiant 225) is manual brakes. I posted w/ photos. The brake pedal winds up in the same place. I used a dust bellows from a manual MC. Another is on a C-body vacuum booster. Another is on a 1990's Intrepid booster, mounted on Dart booster brackets (see avatar). The Breeze MC is good because a tiny 7/8" bore which makes an easier pedal for disks. Insure you get the ABS version (2 ports, same dbl-flare fittings as in old Mopars).
