Indeed it ended up being 1/2" NPT, so I locally purchased a universal aftermarket unit made by VDO, but sure do appreciate your leads on NOS options. Before installing it, I removed the 'T' pipe fitting which has the sender at the top, a plug at the bottom, and screws into the side of the thermostat housing. It was packed full of hardened sediment, so I replaced it also. Then I discovered that the gauge did not work properly. I tested it by grounding the lead which connects to the sender and the gauge went to the cold end - but my understanding is that it should have gone to the hot end. Then I remembered that this truck was a 12 volt positive ground system originally, and likely the gauge was designed for that system. I had an NOS Stewart Warner gauge from the 60s, so I installed it and BINGO, it works fine!
