I have some low profile nuts, which you might be able to find at your local hardware store. I am not familiar with Kurt regarding their bolts so could not tell you if they are metric or SAE. Sorry I could not be of more help.
If it was me I would take my nuts (well, the vises anyhow) and grind them down for clearance. Make sure you hold onto your nuts securely lest you lose them, then the wife will ask you what you are looking for and when you answer that you are looking for your nuts that conversation usually gets you nowhere...