Alan-
Way to go on the conversion. I am not an expert, but am in the process of getting a early 70's Nova drum-drum system in line. You'll need a master that can accomodate a residual valve for the rear drums. However, if the master is a dual chamber design, I do not THINK you need to proportion valve the whole set up. Other experts should affirm or contradict me on this point!
Consider master cylinders from vehicles with similar systems. My window is narrow, but there were certainly 70's GM cars with the disc drum set up. These are available from your local parts stores or other performance sellers, including Chevy Duty. Also, consider calling any of the brake upgrade folks for additional info (such as ECI, MP Brakes, Classic Perfomance, etc.) I had good experiences with ECI and Classic Performance.
Good luck!
