Here's a decent deal on some of the parts you would need:

http://www.corral.net/forums/showthread.php?t=898479

All together there are 10 peices that make up the 87-93 GT ground effects:

1 - Front Bumper Cover
2 - Driver Side Front Fender Ext.
3 - Pass Side Front Fender Ext.
4 - Driver Side Rocker Cover
5 - Pass Side Rocker Cover
6 - Drivers Quater Ext in front of Rear Tire
7 - Pass Quater Ext in front of Rear Tire
8 - Drivers Quater Ext in back of Rear Tire
9 - Pass Quater Ext in back of Rear Tire
10 - Rear Bumper Cover

If you use a Cobra Rear Bumper cover, #8 & # 9 will not be needed cause the Cobra Bumper Cover wraps around the rear quaters.