[rotorwrench]

It's a filter adapter gasket. It should be easy to get. There were several applications but the 67 Merc application is likely the same as the 65 T-Bird. The 390 should have around 26 psi at idle. I would get one that is around 15 PSI. Hobbs meter switches come in different psi ratings. You would need the three terminal type so that the light will go out when the pressure is applied when properly connected.