Depends on your wheels too. The widest tire I can get under the rear of my GTV is a 165 since the offset of the 5.5" Sprintstars is so much. I would LOVE to run a wide XWX, but it just aint gonna happen! Since I am limited in the rear, Ill choose a 145 for the front (135's look too small to me), on 4.5" Sprintstars. This will give me enough clearance for the drop spindles as well (Im glad to find someone else who is not afraid to lower their GTV!!). I am getting Michelin XZX's since it is a vintage type tire, and I must have matching tread from front to rear. The 145s and 165s will fit the 4.5s and 5.5s very well, giving equal sidewall buldge from front to back.
Pirelli Cinturatos would be an excellent choice too, but good luck finding them!!!
I say run a radial tire, its more cal look
What wheels are you using??