I just went through this with my 56 Truck. I have a disc conversion on the front and running disc on all four corners. I changed my master cylinder out because the other one 74 Monte Carlo did not work with the disc on the rear. I used the 67 Corvette master cylinder and it worked great. If the MC is mounted on the firewall, you won't need the 2 lb residual valves in the line. I did put a adjustable proportioning valve in the rear line, Wilwood makes a nice one. That way, you can adjust the rear brakes so they don't lock up, etc. Do a search in here, there are several postings on this subject. Good luck, Merry Christmas.