Rob,
Won't get a chance to reply in full until tonight but once the bolts are adjusted you really need to cross drill it and fit a dowel to stop the slipping. Also make two decent thick washers up instead of the off the shelf punched out ones that will never tighten correctly.

It looks to be reasonable made but the direct belt drive frightens me. It really needs to go thru a worm drive so it can't drive back.
Something like this.