I run 17x9.5's up front and 17x11.5's in the back. I have serious fender rub on the front upon severe spring compression, and my friend running 17x10s has the same. We are both running 275/40/17 up front and 315/35/17 in the back. My rear fenders are cut to accomodate this.
On a Top Secret GT300 kit I bet you could run 10.5 or 11 up front and 13 in the rear.