The sending unit you have pictured is not the one for a stock temperature gauge.
Here is the correct one.
https://www.amazon.com/gp/product/B000C9SM22/
Others have purchased a good one from Lectric Limited, and the size and shape is exactly the same as the one I picture from Amazon.
There is no 3/8" NPT sender that works with the gauge, and you can only use the 1/2" NPT sender with stock gauges.
If you want to put it in a 3/8" NPT hole you need to have it turned down and re-threaded by a machinist. Forum member A1971Blazer turned mine and threaded it, and others have reported forum member brian mac does the work, too.
You instrument cluster looks like a stock one, but Accelo asks a good question, does the temperature gauge on that cluster come with an specifications or is it sold as a direct replacement for stock? Or is it a restored stock cluster?