Personally, I dislike wheels hanging out of the fenders...but for a no-tub/no-flare 12" wheel fitment on the rear of a C5, that's about the best you'll possibly do. I assume you ditched the rear brake ducts?
The rears are tolerable, the fronts are the deal-breaker. I'd look a pair of front repros in a C5 offset. I don't know what's available, but an 18x9.5" in 54-56mm offset would be about perfect, and fit flush with the fender.
