Custom Post Type as Taxonomy or Custom Field

I am working on a genealogical society website that needs custom content for townships, cemeteries and churches. I have created these custom post types. The most common link between the three would be a taxonomy I call 'gridid' (unique to the way Maine townships are organized). However, I would like to associate the Cemeteries to the Townships they are located in.

How can I add a "parent" dropdown to select the Township associated with the Cemetery?