Here's how I've done it for years, we just did one on the kid's car...
The bushing in the gauge kit is 3/8" pipe, you have to find the proper one... Then you drill a 37/64" or 9/16" hole in the water jacket of the intake on the driver's side of the intake manifold next to the thermostat... Then tap it for 3/8" pipe thread....
Be careful when tapping the pipe tap so you don't go too deep... Make a few threads, then try the bushing... Then try a few more threads, and then try the bushing... Repeat until you can get to just thread like shown below. You just want to be able to get the bushing in and not bottom out the threads on it as then the sensor may bottom out on the water jacket...
Drill Size for Pipe Taps
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Here it is from a top view...
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Here it is more from a side view... Notice how many threads I have left above the hole, but the bushing is tight and will not leak... This is what you want to shoot for when doing it this way...
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