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Most likely the radiator is partially plugged due to the block sealer, it may have been already and the sealer made it worse.
Over time the radiator can collect sediment that cannot be flushed out and the sealer will make it worse. To check this get the engine up to normal temp and turn it off and then being careful of the fans which can still turn on, compare the temp at the bottom of the radiator to the temp at the top of the radiator, if it is much colder at the bottom than the top, or there are any areas that are cold, that is a sign it is partially plugged and the capacity is reduced.
If available you can use a non-contact thermometer.
Removing the thermostat allows more flow, that will tend to help if the radiator is comprimised.
Is the cooling system holding pressure now?
Was there pressure on the system after your drive? You can check by squeezing the top radiator hose with your hand.