Confirm with LMC it is intended to be used as a stock gauge and doesn't have a special sender that they sell for the gauge. If it is supposed to read same as stock except it has numbers on the dial, then: I had that symptom on my 1969 truck with very low temp readings on the gauge and I had the wrong sender.
Does your temperature sender look like one of these:
https://www.amazon.com/gp/product/B000C9SM22/
https://www.lectriclimited.com/tempe...ng-unit-117510
Here is a thread where davepl shows us the actual resistance readings the sender should have at various temperatures. You need to use some antifreeze on the stove to get the temperatures of the liquid over 212F, because water on the stove won't get beyond 212F and is not "hot" to an engine.
https://67-72chevytrucks.com/vboard/...d.php?t=714839
If your engine does not have the 1/2NPT hole in the head to accept the sender, they can be turned to and threaded to 3/8NPT or some metric size if you need that, by a machinist. This has been discussed a bit in other threads, have the temperature sender machined down to fit the hole (in the case of my 1969 350 engine with 1990's heads). I had forum member A1971Blazer turn and thread mine. Other people had it done by brian mac. Here are a couple of threads where it is discussed.
http://67-72chevytrucks.com/vboard/s...d.php?t=703388
http://67-72chevytrucks.com/vboard/s...d.php?t=627880