Replace Master Cylinder

I had a 69 Dart with 9" drum brakes. The MC was different, with equal size reservoirs, not the large rear one everyone here notices. That MC was almost always on disk cars, but never say "always" with older U.S. cars. First priority was keeping the assembly line running, so who knows if they raided another part bin. Are you 100% sure you don't have front disks (especially since a GT car)? If you do, jump for joy. If it ever did have front disks, you probably still have a "combination block" for that instead of the simple "distribution block" on drum-drum cars. Search for photos to compare.