I'm undecided on what wheel width I should get. Do you think an 18x8 wheel looks too whimpy?

 

Well it would look that same as the stock tires with the exception that it would be a inch wider. Personally, I wouldn't go any less than 9.5" but finding a wheel with the correct offset is almost impossible right now

 

I would go with the wider 9 or 9.5 wheels, it keeps your options open if you decide to go to wider tires. I think the 8's pretty much limit you to 255's.

 

at least 10-11" in the rear and 9" in the front
That way you can run some meats in the back and some decent 275's in the front.... but then again if you are not gonna add a bunch more hp then the bigger tires = more friction = slower car

 

& more unsprung weight.