The biggest I would go is 275/35/18 in the front. But that will rub when you lock the wheel left or right. So that would be an 18x9 wheel. For the back the biggest you could fit is an 17x11 or 18x10.5 with 315 tires. And If you take out the quad shocks you can fit 335 tires.
If you are going 17 inch. I would recommend 17x9 front with 265x40x17 and for the rear 17x10.5 with 315x35x17
If you are going 18 inch. I would reccomend 18x9 front with 265x35x18 and for the rear 18x10.5 with 315x30x18
1998 Black GT, swapped back to NPI. 281rwhp 318rwtq on E85