You can swap a T5 out of a later model, but i think you'd need a new bellhousing and driveshaft. Lukily, those are highly available. You'd probably also need to get all the clutch linkage stuff like the clutch fork, clutch cable and adjustable quadrant. You can buy some heavy duty stuff from many companies and unlike your current clutch, it's self adjusting.
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Mustang Parts Specialties ripped me off, don't buy from them.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT