You have a 4R70W. The 4R75W came in the 03/04 Mach 1.
If you do the J-mod, you could probably get by with up to 400hp and have a reasonable life out of it if you don't beat on it all the time.
The 5R55S (5-speed auto) is in the 05' and newer Mustangs. Same tranny thats in the last T-bird and...