First!
Do a major tune up of your 87 GT. The Haynes manual and the Equus code reader will be a great help.
Haynes around 10$
and code reader around 20$
Here what the code reader look like
You'll save a lot of money with self diagnosis.
Then spark plug, Cap, Rotor etc.
After think about mods.
removing air silencer, changing the stock mufflers and adding a K&N filter for better flow are some of these.
Hope this help