If you haven't checked this online tool out yet, I highly recommend it. I use it all the time. It's very helpful.
http://www.rimsntires.com/specspro.jsp
Also, I'm not sure if it's intentional, but the tires you say you have are 26.7" tall and the tires you say you want are 28.1" tall. Are you sure the sizes you listed are correct? Your speedometer will read faster with the 18s you have listed.