A good machine shop can convert the more common late-model 50oz. Stock 28 oz. The larger clutch may not clear the inside of the T5 bellhousing. The starters from early 289-302 Fords with automatic transmissions also work, however starters from manual transmission cars of this era seem to have a slightly larger mounting lip and don't fit unless you mill or grind this surface down. Your best bet is to use it as a core at the parts store.
Automatics have a neutral safety switch, so that the car can only be started in park.
For a manual you do not need this, and the switch must be by-passed or the motor will not start. On early Mustangs the four-plug harness sticks out of the firewall above the master cylinder. Two of the wires control the reverse lights, the other two control the neutral saftey switch. On other cars you can check the wiring diagrams in a shop manual, or trace the wires from the original automatic shifter. The backup lights can be made to work by connecting the other two wires to the two leads on the T5 case.