early a master cylinder swap

The "combo valve" from the 67 drum-drum car is just a "distribution block" with an "imbalance warning switch". You don't have a dash lamp for the later, but otherwise it will work fine.

As many have, I simply used the distribution block in my 64 & 65 cars. I plugged the "rear" port" and installed a 1/4 to 3/16" inv flare adapter in the top port. I plumbed the "rear brakes" MC port (fwd port) to my existing rears tubing, using an in-line coupler. Search posts for Autozone PN's of the fittings needed.

That MC will work fine if you later change to front disks, but you will need a proportioning valve on the rears tubing. An adjustable one is cheap on ebay ($30). With manual MC, you may prefer a smaller 15/16"D or 7/8"D bore MC for front discs. I use a modern aluminum MC w/ 2 to 4 bolt adapter plate ($30 ebay).