Sounds cool. Would love to see pics. I have no idea on the 18" part but the 4.5 back spacing should be no problem. A friends truck had 5" back spacing on it. It was tight but it worked. These were 15" wheels though. Personally I think you need more outward spacing than back spacing like an 8" wheel with 3 to 3.5 back spacing would look better in my opinion.