The option GT wheels are 17x8" wheels. My GT came with the *beloved* Tri-Bar 17" wheels. The stock tires were a 245/45R17. I currently run 275/40R17 on 17x9" wheels and I think they are a good fit. I would think that 275's would undesireably bulge on a 8" wide wheel. IMO, I wouldn't run any wider than 255's maybe 265's on a 8" wide wheel.
Cool, cool. I saw a 94-95 Cobra, with factory wheels running 275's on the rear and it didn't look bad to me. Are the factory Cobra wheels from those years 8" wide also?
I ran 275's on all 4 of my original tribars and it looked good but the handling went down a bit due to the sidewall angle. The installer also had a hard time mounting them. 255's are a much better fit for the stockers.