thank you all for your help, my water temperature saga is coming to a close. I ordered the auto meter M-16 x 1.5 adapter, which got me down a size but is intended for a water temperature gauge that's mechanical. This is for a capillary tube set up.
I ran in circles for a few days trying to adapt this mechanical pressure fitting to a electric sender that auto meter supplies with the gauge. Unable to find anything that would fit that size, and about to capitulate and drill a hole in the thermostat housing, I stopped by Napa, and they had a pressure fitting that was double end threaded.I simply chopped off one threaded end, drilled and tapped the rest of the pressure fitting to 1/8 size and screwed that into the auto meter adapter. so far no leaks, fits well, and looks okay. I'm keeping my fingers crossed.
I don't understand why this is so difficult, because it seems like going from metrics to aftermarket gauges would be a common event. Most people looked at me and scratched their heads.