Stock 98 GT wheels, so I believe 8". I've read some places 255 max, then others they say 275.
If they are stock wheels for the GT, then I believe they are 7.5" wide (the Cobra's were 8"). In either case, 255's are probably too big and 275 are for sure too big. For that size wheel, for a proper fit, you should be looking at 245's max.
Now, having said all that, you can just about put any size tire on that you want. 255, and 275 for that matter, will go on the wheel, however the wheel doesn't have enough width to support the sidewalls. If you DD your car, and drive like a grandma, then you *might* not ever have any issues. I do not recommend this though. Instead, if you want to run wider tires, then get wider wheels. There are a lot of affordable options at American Muscle and Late Model Restoration. If you need tires now, get 245's, save some money on the smaller size, then put away some money for new wheels/tires.