No but it seems you do not know as well, as you have provided no answer.
I'm sorry, my response wasn't meant to be mean spirited. Since you are a founding member, with a documented true 66 Fastback GT car, I thought you were kidding around.
The GT option included stiffer springs, larger sway bar (as mentioned above), better shocks, quick ratio (for the time) steer box. It also included manual disc brakes, dual exhaust through the valance, GT stripes and badging... I *think* that's about it. A good search here, or through a couple of Mustang books (like Colin Dale's Original Mustang 1964 1/2 -1966, will get you a full answer.
Man, that original window sticker showing your car as a "true GT" makes the car even more valuable. If I could please suggest having the car looked over by an MCA concours judge before you start making changes. You may be better off to sell it, and buy a nice C code. You'd be way ahead money wise.
It's your car, but as a fellow enthusiast, I'd hate to see you do something irreversible that you'd kick yourself for down the road.
Scott