The spacer goes between the shifter and the mounting plate. My shifter is not adjustable in position like you described. I think yours is not original? It does sound like you are still in a forward gear when you try to put it in reverse. This does not explain why your shifter will not go all the way to the left. That sounded like the spacer that holds the shifter away from the plate so that the center pin can stick out the side of the shifter housing/assembly.
Jim