I just a 1986 Mustang GT. I was told that I couldn't add any after market parts to the motor?
You were told wrong. you may need to upgrade to a later model ECU (computer) with a Mass Airflow Sensor, but these parts are available in junkyards & from Ford and many other aftermarket vendors. Congrats for buying one of the best looking GTs in the '80s. The '87-93 models were cheesy looking IMHO.
There are TONS of aftermarket parts for these cars, everything from mild to WILD ($$$). I suggest you pick up a 5.0L Mustang enthusiast magazine & familiarize yourself with the car & what's available for it.