My Power Disk Brakes are frustrating me...

I went through the same thing, too big of a cam, not enough vacuum. First I tried a master power brakes vacuum pump with a tank reservoir. Unless you down shifted to keep the vacuum pulled down coming to a stop you had maybe two applications with the booster in play. The vacuum pump sounded like a wounded cow sitting at a stop light. I ditched the booster. Than replaced the master cylinder with one for a 71 demon w/340 4speed. non power disk and standard rear shoes. dropped right in. This worked so good I did the same thing on my 64 fury when I installed disk to the front!! Don't forget to get the right prop valve. Hope this helps, good luck.