You either have a T45 or 3650. Probably a T45 if it came out of a 98 but they are interchangeable. Look up pics on google of both transmissions (do NOT google "tranny" actually type transmission...) and see which one you have. If you can't tell from underneath for whatever reason pull your shifter bezel and take a pic of the shifter. The t-45 uses the same shifter as the t-5, the 3650 will look different.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.