I don't think there's an alternative trans that will fit. I had to replace the trans in my 89 T-bird with a used one from a junkyard. These transmissions seem to go bad a lot so they're hard to find used. I still have the Bird but it's been sitting out back since the doorpost rusted off at the bottom and caused the windshield to crack. It's a good parts car but I'm too far from Bismark to do you any good. It has to go soon.
My aerostar van has the same engine and transmission but I haven't been under there to compre them to see if the case is the same. They also have a history of tranny trouble. But it's worth checking out if you find one in a junkyard.
I don't know if the T-Bird supercoupe had a 4 speed manual trans but if it did you would need all the clutch linkage too.
If you have to give up and junk the Bird save the rear end. It has independent rear suspension in case you didn't know. It might be a good fit in your 49 Plymouth.