240* is really hot there bob, careful running it for that long. May be helpful to pull the lower radiator hose and check for junk that comes out (from the radiator). If there are signs of stuff you may be able to clear some of it out over the course of a couple days. What I have done before is sat the radiator on its back and filled it as high as i could with a full bottle of "coolant flush" and water, and basically just let it soak for a few days before washing it out. Worked decently and was cheap..
It also wouldn't be the first time a faulty "new" thermostat was sold. That is a possibility as well.
I ran it to the car show without a thermo (and had been without one the prior week for testing) and it still overheated....
There is a thoery behind running no thermostat
can cause overheating in some vehicles. The system is designed to have that restriction of coolant where the thermostat is. The restriction helps keep the coolant in the radiator long enough to disappate the heat, coolant flowing too fast will no be able to lose enough heat. 'Possibly'. This was the case with my '70 Dart at one point, with no thermostat.