FYI, I friend of mine is finishing up a restoration on his Mustang II "King Cobra". It came with a factory 4-speed and he was unable to find a clutch for it. He had to send the factory clutch to a specialty vendor who rebuilt it and it was pretty expensive.
That same friend also has a Mustang II Cobra II with a T-5. It all bolted in just fine. I've driven the car and that's the way to go!
The original 4 speeds in the MII cars are very weak. I would also suspect they are hard to come by. I had trouble finding one 30 years ago when I owned a 77 cobra MII with a 4 speed. I ended up installing a T5 in mine and a later a very healthy built 289.