Hi David,
Very sorry you found this unsatisfactory. I think it may just be a simple misunderstanding. The spacer kit is intended to add additional height to the mount, so it can clear in instances where the cab of a vehicle or additional accessories (light bars, etc) require. The Top of the bolt is unthreaded, so a set in nut would prohibit it from functioning.
It sounds like you may be after a different end goal. If you can tell me more about the mounting consideration you are trying to solve, glad to troubleshoot and get you the correct parts to get mounted and secure. We are always here to help!
Best,
Rick & the Riversmith Team