John,
Did you look at the body assembly drawing (AM0016, pg52)? It shows no sealant as that spherical washer is used to center the force of the nut on a surface that is at an angle. As much as I would want a sealant in this application, Ford did not. (It's the same spherical washer used on back-up lights for the same reason, an angled surface.)
OT (sort of). It's been a while since I reviewed a 67 Fastback rear vent assembly. I note there is a drain hose again on the rear vents after being introduced in 65 Fastbacks, and deleted in 66 Fastbacks. Just an observation.
Jim