Excellent. What a frustrating search this turned out to be. Now that you said "5/16" steel fuel tubing has 1/2-20 threads on the nut", I remember it swapping a fuel pump on another vehicle. Why the hell I thought it was a hose barb, I have no idea. Went to AutoZone, nothing in stock. Had them look up the pump, couldn't tell me what thread it is. Spent 2 hours driving around to no avail. Anyway, thank you.The tapered face in the back of the hole identifies this as an inverted flare, like steel brake or fuel line.
5/16" steel fuel tubing has 1/2-20 threads on the nut.
Original application would have had a steel hard line, bent 90° as close to the nut as possible, so that the line goes up the front on the engine in front of the passenger side head / behind the alternator.
If you want just a hose barb, it will be 5/16" male inverted flare to 5/16" hose nipple adapter fitting..
Any place that makes hydraulic lines should have the fitting.
Any auto parts store decently equipped should have also had it......but things today aren't what they once were as far as inventory on hand....