I'm back home from my travels and had a chance to look in the Ford Parts Book today - Per Ford the early C6's ( I'm going across all engine families here ) from the 1966 model year until about the 1969 model year used the 1/4-18 thread fitting. Then starting around the 1969 model year some of the cases started using the 1/8-27 thread, and some kept using the 1/4-18, I see an application for the 1/4-18 thread up through the 1972 model year. From the '73-'76 model years it looks like they all use the 1/8-27 thread, but then starting in the '77 model year to at least '79 they start using the 1/4-18 again. I don't have the Ford Parts Books after the '79 model year so I don't know if the 1/4-18 thread continued to the end of C6 production. So there are two possibilities, but it's really a mix when they were used. I didn't check every application, but my quick observation was that Ford specified an o-ring on all of them.