I don't see much of an issue with this at all... 245s might be a bit wide, even for the wider 8.5s, so if you can find some 235/40s I think you'll be in much better shape.
Toxik had the g37 replica 18s on his car. They measured 18x8s and he used 225/45 tires on his and they fit really nicely. 40 is a good sidewall size (I imagine the 225/45s might have looked a bit thick on 19s), but I'd have to say my recommendation is 235/40-19 all around. With the front tires, you might have a bit of swelling over the lip of the wheel but I imagine the rears will be pretty flush with it.
http://www.rimsntires.com/specs.jsp
Use this website to check your diameters and tire sizes if you want. It's a pretty useful tool. Word!