Instead of the distribution block you should use a proportioning valve or an adjustable proportioning valve for the rears.

I have a 66 charger with swap had the same problem it would stop the car but it would not hold the car back if i tried to brake torque put on a new dual diaphram booster from murrays for 108.00 worked great but i did have a proportiong valve. had to rework the lines a little cause the dual diaphram booster is longer. Whats the bore size of the master?