The rear upper and lower control arms on all 83-88 T-Birds and Cougars are different than Fox Mustangs. ( i.e. longer)
79-98 Mustang control arms will bolt on,
but will not fit properly.
The 83-88 T-Bird shocks are also longer than a 79-93 Mustang, but the SN95 (94-98) Mustang shocks are the same correct length to use on the T-Bird.
This is what Mustang control arms on a T-Bird does...
moves the rear forward about 1 inch.