On the back,a 17x8 fits nicely with a 5 inch back spacing. You should be able to keep the quad shock with the right tires. If you remove the quads and roll the fender you can tuck a 17x9 or squeeze a 18x9 with a 275 to 295 wide tire. The front depends on the year. In....90 I believe,they opened the wheel opening. Pre 90 a 17x8 with 5 inch backspacing fits alright with no bigger than a 245 wide tire. Post 90 a 17x9 will fit and again a 18x9 with a narrow tire.
I have an 88 GT and i have Cobra R's on it. The backs are 17x8's with 255/45/17's and they rub a tad when I hit hard bumps. The fronts are 17x8's as well and they also have 255/45/17's, there a little to big for the front. They rubbed alot when I first put them in but i got most of the problem fixed now. I am not sure what the backspacing is on them.