I just mounted the stock potenzas on other rims (same exact specs though...18" x 8" with a 50mm offset) and they fit fine. BTW, they are wedsport sa70s. The outer edge of the rim sticks out a bit from the tire but it's not too bad. I'll post a pic in a few days when the weather clears up.
I hear ya there and I've been going back and forth about going 235-40 or 225-45.
In theory the 235-40 is closer to the stock tire size. After plenty of reading through the forums I've decided to order the Falken FK452's as the Falken's appear to be a thinner tire than others.
I'll be able to tell for sure in a couple of weeks ... if I have to roll the fenders, that's ok too.