A 245 tire is just fine on a 9" wheel. In fact, I think it looks slightly more correct than the factory 8" wheel. The sidewall is streight, more like it should be. Look at lower profile BMW wheel and tire combinations to see what a tire looks like mounted to the correct width wheel. The origional ford 8" wheel is toward the narrow side of the range.
Yes, compared to other mustangs, a 245 on a 9" wheel will look slightly small and leaves the rim slightly exposed, but it is perfectly fine.