without seeing an actual photo. I would guess the one thats 18" farther has a drive shaft and not a coupler. There were probably several designs as everyone wants to find a better way to go faster, and handle better doing it. It probably would be a newer design rail if it did have a shaft. If you are wanting to run in a true nastalgia class you might want to check the rules for the engine placement and see if it will pass thier tech.