Master cylinder interchange disc/drum-drum/drum

Yes, I still have drums all around, but prepared for front disks by installing a newer MC w/ booster in my 65 Dart (avatar). Many here use a manual MC w/ disks and are fine w/o boost. The pedal rod in my 64 Valiant (no booster) fit fine in the Breeze MC. The rubber bushing at the end locked tight in the piston. Search for my post w/ photos. The pedal wound up in exactly the same place. I super-glued a bellows to the new MC as a dust shield. Most 1960's manual MC's come w/ a new pedal rod & bellows, as you can see on rockauto. If re-using your pedal rod, you will need a new rubber bushing for the end. I recall buying at one of the specialty places (Dr. Diff) or perhaps ebay.

The biggest change when you convert to front disks is to install a pressure proportioning valve in the rear circuit, to reduce pressure. You can buy an adjustable one for ~$30 on ebay (or Summit). Without that, rear drums will lock up first, causing spin-out on turns. If you also install rear disks, you might skip the p.v., assuming you size all the calipers correctly (hard do do w/o an R&D team).