[HRBS]

Doug....
I've done this a few times on Novas.
18" wheels fit perfect.
I'm currently building a Nova with 19's and I think this is max with the stock openings. Attached are pics of what we've done when running 20's.
I basically got a fender opening patch panel and split it in half.
I widened the opening 2" foward and 2" backward for a total of 4".
I also got a rear sail patch panel and used it to "pie" cut the quarter so it matches up and the bottom. Hope this helps.