Im pretty sure its not as easy as one thinks to set clearance on the adapt. If your in need of less belt to sheeve, you actually have to machine the bearing upper collar to do this.
I do not know how these can be shimmed either, the spider base that threads to the post is an odd design and simply adding shims will do nothing but adjust belt clearance.
Only thing i could think of would be to make a shim to put under the actual spider between it and the base. But you would lose strength being there would be a space between the 6 bolts holding it to the fixture.
May need a custom fixture machined with certain amount of shim increments that would attach back to the spider with the 6 bolts