I just got done doing this on my 1964 Dart. I converted to 1973 Disc brakes and went with a rebuilt 1973 master cylinder. if you are keeping the drums get a cylinder designed for drums. I used a proportioning valve made for 1973 Disc brakes.
the one issue I ran into was the front lines on the car had a different size fitting than the new proportioning valve. the lines were the same size but the threads on the fittings were bigger. I'm not sure about a drum brake proportioning valve. I capped off one of the front line outs on the proportioning valve used a short jumper line with one fitting of each size on either end and a brass tee. I hooked both fronts to the tee, and the tee to the jumper. the rear hooked up as stock. by far the easiest way to go, trust me I thought about it a lot. the jumper I used was a factory piece about 6" long with one big and one small fitting and the proportioning valve already had one side capped off, some of these must have come like this so look for one. be carefull to get an actual tee, the one on the stock rear brakes is actually a pressure valve to keep fluid up at the rear, I started with one of these and as soon as I hit the brakes it worked like a line lock.
if you need I can take a photo or two but it's pretty straight forward.
