The GT500 runs a 9.5" wheel up front, but with a 255 tire, as I recall. I bet you could go with an 18" up there with the same off-set and install a set of rack limiters to avoid rubbing. I don't know how much more tire you can run in terms of width though. I know the fenders are rolled, so you may be able to run a 285 if your sneaky.
Do some research on the Shelby forums and will find out some additional info. You will find a link to one in the S197 wheels section of Choosing a new tire link in my sig.
Jazzer
