I had that symptom on my 1969 truck and I had the wrong sender. I understand 60-72 and perhaps a couple years beyond that required the same sender for the stock gauge to read correctly. You didn't mention if you have a stock gauge or what year you're dealing with, perhaps 62. Are you dealing with stock gauge or aftermarket? If stock gauge, does your temperature sender look like one of these:
https://www.amazon.com/gp/product/B000C9SM22/
https://www.lectriclimited.com/tempe...ng-unit-117510
If you have a more modern engine that does not have the 1/2NPT hole to accept the sender, they can be turned to 3/8NPT by a machinist. This has been discussed a bit in the 67-72 section, have the temperature sender machined down and re-threded to fit the hole (in the head in the case of my 1969 350 engine). 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