I spoke up in your other thread, so I thought I'd speak up here.
The last "Fox" car I saw with 28" tires on it was actually a Fairmont. Its a killer 9 second car on the juice. I believe they run both 26" and 28" BFGs on the car. With the 28" mounted, the car looked absolutely badass, but I believe they had pounded the hell out of the fender to fit those tires, and there was still hardly any clearance.
The 28" tires definitely give the car a mean look.
We've had more tires on these cars then I can count. What wheel and backspacing are you looking to run? My dad has had anything from 26x9s to 28x11.5s on his, even 275/60s etc... Not a GT but if I knew I could shed some light on how it fit on his.