Moving it back 18" will make the truck a 6500 Series (179" wheel base) and will take you from two drive shafts to three.
If I was doing it I'd cut the frame in the tall section behind the carrier bearing cross member and add the 18" there. Others more knowledgable on this than me will tell you exactly how to cut it, weld it and plate it (if you need that help).
You may need another cross member as well.
Mike B
