Jerry,
It looks like your recollection is correct; the threads in the head seem to be 5/8x24. I don't have anything to test them with at the moment. I have not found what appears to be the correct adaptor out there from the vendors yet.

I have attached a few marked up photos.

One of the photos shows the temperature sensor bulb that is attached to the gauge in the (original?) dash. The threaded adaptor on that one is significantly too large. Any idea of what engine might have been in this truck that would have used that?
Attachments
Temp Sensor Port Marked Up.jpg (292.04 KB, 88 downloads)
What Was in the Hole.jpg (182.22 KB, 87 downloads)
Came to Me Disconnected.jpg (171.14 KB, 87 downloads)

Nick
1940 KC Model 1/2 Ton Pickup Project
(with '37-'39 Bed and what we think is a '42-'46 Frame)
Taos, New Mexico