That's a drum brake (9") master.
You need a disc master, like the Centric 13063019... bore size will depend on your particular taste for pedal feel, as well as whether you're running a power booster.
For manual brakes people seem to like a 15/16 bore, like the Dorman M80266.
For power brakes it's more in the 1.031 range., lots of suppliers for those (like the Centric unit I listed above).
To keep your preferred "period correct" look, browse through all the listings for Mopar RWD master cylinders through about 1977 to find the bore size you want.
I'm using Rock Auto's listings just for reference.