Unless you are boiling the water out of the engine, I suspect you have a bad temp sending unit. Your temp should be running about the same as the rating of the thermostat, most are in the 160 to 180 degree range. Any temp over 220 can be harmful to your engine. First thing to check is the temp of your water. Loosen the radiator cap so it will not hold pressure. Start the engine and let it warm up until the temp is stabilized. Then remove the radiator cap and insert a thermometer that reads in the 160 to 240 degree range, I use a meat thermometer and some candy thermometers will work. When the water is flowing get your temp reading and compare to the dash gauge.

Another possibility is that you put the thermostat in upside down. Spring goes down, on the engine side. If these aren't the problem, you're looking at something being plugged, probably the radiator.

Fred
52 3600
69 C-10