Order a modern master cylinder and don't look back. Dr. Diff sells them with the adapter. The pushrod from a a body manual brake car are spot on for these master cylinders.

56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d