It does not need to line up exactly like the chain does. So long as the belt rides near the middle of clutch wheel it is ok.
I think your using the 40mm belt Tom?,
The baskets are all the same but the 30mm belt (for road bikes) sits closer to the back of the basket, you probably have about 1-2mm of basket that you can see at the back of the belt, and a whole lot more at the front side of that.
The spacer comes in 3 sizes I thought, but I may be getting confused with Layshaft spacers. Definitely more than one size though.
I beleive I am on about item 5 on the diagram, I don't beleive these ever came in more than one size.