which is easy to solve with a minor fender roll.
Also looking at 225/50R18 tires.
i ran the front off the gxp on the rear of my w body and the rear on the front
run the 18x8s all around the car. the 18x7s are too skinny for an 18" wheel.
many people have done it, youll have no problems.
to run the same tire size around you dont need to do anything. you can run the same size all the way around with the current wheels.
however, to do it the 'right' way you should purchase another set of GXP wheels and reuse one set so your wheels are the same size all the way around. that way the tires all fit the same and you can rotate them all correctly. then you could also use the second set for snow tires in the winter.