Master cylinder questions

Kosmic is right. But you could keep using your same master cylinder if you could find a manual brake pushrod -- the master cylinders are completely interchangeable between power and manual, and yours should have the 15/16" bore which is preferred for manual brakes. Just remove the booster. Factory manual brakes used a 1-1/32" bore which made for a very heavy pedal. The 7/8" bore Kosmic mentions is a size used in later models and B bodies, and it would work perfectly fine as well -- the Mopar disk brake MCs are all basically interchangeable.