Just how hot is it getting? Does it boil over or is it just the dash gauge saying it is hot? Sending units and gauges can fail. If it actually is overheating do not allow it to get above 230 degrees or you could blow the head gasket.

First I would try running it on a test run with the thermostat removed. Just as a test to eliminate the thermostat as a possible culprit. If it still overheats, remove the upper hose from the radiator and start the engine. Rev it up and watch carefully how far it shoots water out of the upper hose. If it just dribbles you have a circulation problem. Even though it had good coolant in it there could have been sludge in the system that has settled in the block and solidified, I have also seen the impellers of the water pump completely rusted off of the shaft. I have also seen head gaskets fail during long storage Just some things to try. Good luck.

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"Without Fleet The Pride Doesn't Ride"

1983 300SD 609K + still going strong!
1970 Monte Carlo
1994 Ford F150