Originally a press in heater nipple I guess. Some Ford pumps are like that. Sounds like you need the smallest pipe plug tap (tapered NPT) that will cut threads plus the pipe plug of your choice. Drill out only if needed. As several have stated, a good hardware store will have the taps, home improvement stores have the smaller sizes as well.

HF (gasp!) has pipe tap sets with sizes 1/4" up to 1 1/4". I have one and for my occasional light use it is fine. I actually needed the 1 1/4" and nobody else in town had it. The taps should indicate proper drill size if needed. Cost is reasonable.