If you retain your drum brakes you don't need a proportioning valve. A proportioning valve is used on a drum disc system because disc brakes require a lot more line pressure to work. Drum brakes are self acting and require a big honking spring to retract the shoes once applied, not a lot of line pressure. This is why most disc brakes are power assist to keep Granny Goodwrech from getting a run in her stockings from standing on a non-power assist with both feet on the pedal..
There are many brake kits for your car. Price affects quality and safety of the parts.
The cheaper kits use parts from the bad part of Asia, Pakistan and India. They are to quote a phrase unsafe at any speed, but they are cheap.
Medium priced kits use brakes off of a G-body from the mid 80's. Those kits don't tell you that you can find these parts at many auto stores as they want you to buy their expensive kit. Look on line for a complete parts list. Once again the cheaper the part the more dangerous it is.
Finally there are the premium part kits made out of billet material by NASCAR racers turned auto parts resellers (Baer and Wilwood). They all offer from stock to huge rotors and caliper designs. From nine inch rotors to over fourteen inches in diameter, with one piston floating calipers, up to six piston bridging calipers.
As to a power assist master cylinder Rock Auto dot com has all of the replacement stock parts that Chevy used (eleven inch vacuum diaphragm, activating rod to fit your master cylinder (there are four different types) and of course the master cylinder sized for drum brakes.