Need help with size, thinking 18's on a 68 dart

I have 15x8's with 5" BS on the back of mine and run a 1/4" spacer. This is with offset hangers. 10.40 section width tires, 26" tall. This combo is the absolute max width wise you'll get under there. Depending on the tire pressure and cornering speed or weight in the vehicle they will scrub the inboard side of the fenderwell and or leafs. There isn't much room to the outside lip either. Maybe 3/4".