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There will be a hole in the firewall somewhere, that probe will fit through. You can find the NPT fittings at the hardware store to thread the probe into your engine. It's been done for many decades.
The gauge kits usually have some adapters for the probe. But you are correct, they do not give you as many as they used to. So you might end up buying a adapter if they didn't give you one to fit. These adapters look like pipe bushings, but they are not. They have pipe threads on the outside, but have a seat on the inside.
But you certainly can use the adapter they give you, and then use a regular NPT pipe bushing to adapt the adapter to the engine threaded hole.
Besides the crappy factory gauge going wonky, how do you know it's overheating? You said there were no other signs. Are you going ahead and spending a bunch of time and money on it because of what the iffy factory gauge tells you? I think I would get a second opinion with a infarad thermometer or a aftermarket temp gauge.
This was my first thought gauge TU? If no sign of over heating try the temp sender about $20 Or maybe the voltage regulator on the instrument cluster if all the gauges are messed up?
Okay here’s the update. The entirely new coolant system is installed and leak free! This also included a new timing chain and gears because I was in there anyway and there was quite a bit of play in the old chain. What a relief and man what a job! I couldn’t have done it without my friend Harry and all of you with such great advise. The stock temperature gauge even works. We had to remove the distributor cap to replace the thermostat. I believe when we reinstalled the distributor we advanced the distributor ever so slightly and man does my truck run smooth. It’s like a completely different truck…I’m super happy. I love these bull nose trucks and hoping to have mine for a very very long time. I get a lot of compliments on it. Anyways I’m off to pheasant hunt with my buddy Harry and just wanted to thank all of you for your never ending support with my truck…thank you. Enjoy your day and Merry Christmas.
Larry