Plugging Smog Hole (thermactor plug) 5/8-11
Trick flow sells them as TFS-51400265 or spark plug works also, or Ford part # F4ZZ-6E086-A head inserts.
Trick Flow Specialties TFS-51400265 - Trick Flow® Small Block Ford Thermactor Plugs - Overview - SummitRacing.com
The threaded hole...