I own a 1972 Skylark custom with front drums and want to convert the to disc. My question is would i have to get the proportioning valve or just the master cylinder?? and what other cars should i look for that use the same setup other than Chevelle??

You may have a proportioning valve, but if you do not, get one. They are relatively cheap. You can even get adjustable ones. If you do not have a dual system, get a new master cylinder, and plumb it for a dual system...front and rear divided. I do not know what else you would use for the spindles with disk brakes.