Have you looked on the American Muscle website at wheel and tire kits? Good prices. I priced tires locally and it's not a whole lot more to just get a wheel/tire set from them. Will make a big difference in the "looks" department of your car.
I have 18x8 with 255s all around...not as wide as I wanted but they seem to work I went with bfg gforce comp 2 and they seem to hook up pretty well as long as I don't dump the clutch anywhere past 3k...my 05 gt is procharged too if that helps any. I would recommend getting wider Wheels and tires if you plan on tracking/dragging it but mine is a DD and it works even though I drive it hard. Hope that helps.
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