First, I don't think 315 are too big. But,if you do, Nitto is going to start making a 305 18", I think. Check that out.
Second, who's making an 18" x 10.5" BULLITT wheel? I know www.wheelreplicas.com has an 18 x 10 and a 17 x 10.5, but no 18 x 10.5. Please fill be in, as I'd love to get some.