[Vern]

If you want to save the gauge, Don’t cut the lead going into the radiator neck. It’s a hollow tube that has a bulb on the end inside the neck. If you cut the tube you will have to get it repaired which is not cheap, but can be done.
If you run the car enough to heat it up, try to loosen it then.
If all fails, Williamson’s Instruments in Arkansas will possibly be able to do the repair to the gauge