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I have a 70 Challenger 440 six pack and have front disc conversion kit by MBM brakes. It looks like the factory Bendex style. They don't sell direct but you can get one of their boosters from Pirate Jack .net. I had to replace my master cyl and they were very prompt on shipping and on some tech questions I had.
I would try to rebuild what you have if its the original. If not, Rockauto has rebuilt ones by A1Cardone. $64 + core. NAPA and other auto parts stores should be able to get them. Or get a rebuilt and keep your original.A-1 CARDONE 5473701