Just to add to some of the comments, since I recently did swap a 88 engine for a 90 angine, as well as the dasboard and other stuff; the distributor is different after 90, cause it was the same when i swaped. however, EGR valve and sensor are different in late '90 models and up (it has a hose that goes from the egr valve to the sensor). Front bumpers are diferent molding, and the 91-92 have some sort of lil' canals for air fluctuation. The wiring looks alike, but if you install the wiring on older versions of the GT it will not work, same viceversa.
The insides, I know the trim of the door panles is different (solid on newer models, stripes and contrast on older ones) and also most of the upholstery. Now, I dont know about this, because ive only seen it once, but I THINK the 90+ models had electric seatbelts (whic i personally hate, cause they attack you willingly) and the older models dont. The frame for the dashboard is a lot more beefier and heavy on the 90+ models, and the mounting of the speakers is on the metal, compared to the plastic of the older models. Also something I noticed on the 91 donor car and not on mine is the the instrument panel/gauge thingy (lol) has a boost meter on the speedometer. Mine ('88) dont. This I'm not sure again, but corrections are always welcomed.
Thats pretty much all I know. Sorry for redundant info and any misleads of info
