[Coley]

It looks like you need to investigate and determine exactly what you have.
They say the hole is a 3/8" threaded...and your part with a 3/8" threaded end doesn't fit. Can you find another 3/8" (Npt-pipe) part and try it to make sure that it isn't the AC delco part you have?
That said, you need to first determine that the hole is threaded...it might have missed the 'tapping' operation when it was getting made so it may be just the pilot hole. Use a small mirror or your fingers to determine that there is indeed a tapped/thread form in there.
If it is tapped you need to try a few different threads in there to determine exactly what they tapped in there....if they tapped it.
Can you feel a thread form in the head?

Coley