George,
It's mostly just a matter of torque and HP ratings. The tremec 3550/TKO are stronger than your stock Mustang T5. If you are using a mild 302, a Mustang T5 would probably work fine. You can always upgrade later if you go to a higher HP 351. Also, remember these cars are fairly light, and usually run street tires, which is helpful to making a transmission live longer.
Good luck
P.S. There are MANY transmission posts here, you may want to go back and read through them.
Dave