I don't think you need to go as far as threatening to resign. I'm sure we could find another venue if the museum says "no". I know a stone aggregate merchant who might be some use in sourcing a 10 ton stone (I lent him an adze last year to dress up an oak beam for his house so he owes me). He knows all the farmers in this part of North Yorkshire and would probably know if any would be "game" for the project.