Stock size should be about 245/45/17. So you are going a little wide with a lower profile. Not gonna fill out your wheel wells too much though. Hope your care is lowered. They will fit, but are they the best look, maybe not.
Are they the T1R's? If so, they are a great tire. I've got the T1R's in a 265/40/17 on a 17X9 rim. You'll lose some tire height on a '01 GT which may give it an even more 4X4 look if your height is stock. The Mach's are alittle lower from the factory so my tires still look all right. I paid $143 a tire for mine last year.
That's the size that I have on my 98 GT. Fit fine. Although I would recommend lowering it, if you haven't already. There's a plenty of gap between the tire and wheel well.