Wayne, whether or not it will look weird might depend on how picky you are with little details. If you see something and it bugs you forever, you might want to steer clear of this combo. It will only be about a quarter of an inch difference on the sidewall. My Mustang came from the factory with 27.4" and 27.9" tires on 19" and 20" wheels. If you look closely you can tell, but most people probably won't notice. I only noticed the front tire had more sidewall a week before I put new tires on. Google 2011 GT500 with SVT performance package. Better yet, see if you can find a new one at a dealership that you can swing by. You probably won't be able to see the difference in sidewall height in pictures.
Or, you could just save the energy spent thinking about this and buy 245/40/18s
Matt