Is your sender an original 1956 Delco? The aftermarket units for 1956 have incorrect impedance and show hot when the coolant reaches about 185 to 190 degrees. If you measure an aftermarket unit with an ohmmeter, it will read about 500 ohms at room temperature. An original 1956 Delco unit measure about 650 to 700 ohms at room temperature. I found a used Delco sender from a 1956 truck in a wrecking yard and my gauge works correctly now.
Get an under dash gauge until you can find the correct sender. You will find that you are not overheating.
I hate when the coolant spews out after refilling the cooling system. You had an air/steam pocket. Drill a small hole about a 1/16-inch in the thermostat to let the air/steam escape. Otherwise, if air is trapped in system after refilling coolant, the escaping air/steam pocket will spew coolant all over the place when the thermostat opens. I learned this after it happened to me. The original style thermostat had a butterfly valve that was not air tight, so spewing coolant is not a problem until a modern airtight thermostat is installed
