getting an 18" rim shouldnt because the tire height should be the same. so a 17" rim and an 18" rim will look have the same overall tire size. this is why 18's use a lower profile sidewall...
so a 245/45x17, a 275/40x17 and 265/35x18 should have about the same tire height.
804: here are the links to each...theyre found on stangsuspension.com
MaximumMotorSports Front Coil Overs with Springs
MaximumMotorSports Rear Coil Overs with Springs