Can I use a 94 t-5 in a 89 Mustang?

The only difference is the input shaft and bearing retainer. Driveshaft length and the casing of the T-5 are the same. Your cheapest alternative is to use a fox input shaft and bearing retainer and all of your existing parts. Your other alternative, is to change the BH & clutch fork to a 94-95, and possibly shorten your D/S since you will be moving your trans back 5/8". You can not use the fox BH with the SN-95 input shaft as your transmission will not bolt up flush to your bell housing.