...When I slowly opened the cap on the expansion tank and promptly got about a good quart out of it before it stopped running.
Just to make sure I follow: when you opened the cap a quart of coolant came out?
A conclusion I
think we can make: the system did not self-bleed from being filled, you had to run it to get more air out (based on the fact that the header tank level dropped after you first filled it.)
Given that, possibly that bleeding action was not complete, so some remaining trapped air in the system heated up, wanted to expand, and when given the chance to by your opening the cap forced a quart of coolant out. But the question that leads to is "where is that air trapped?" Regardless, at this point I would be tempted to give it another chance or two to bleed off any air; perhaps jacking up either end while its running to see if you can free up some trapped air and/or seeking out steep hills.
There are chemical-based kits for testing for head gasket leaks into the cooling system (Napa has them). I have one which a friend has used but the results seemed a little inconclusive to him. One sign of a head gasket leak that I have experienced is that the spark plug in the cylinder with the leak will be distinctly cleaner than the other seven. Also there will be a smell of antifreeze from that cylinder with the plug out (assuming of course you're using anti-freeze). And maybe even a drop or two of water, which is distinctly different in appearance from any other fluid like gasoline or oil.
You can investigate the clogged radiator theory by feeling or measuring its temperature over its surface with an inexpensive infra-red thermometer/pyrometer (just hold it close to the radiator so you don't get tricked by the width of its measurment beam). When the coolant is reading at ~200 F the radiator should be around that temp over its full surface. If not then it's not doing its job for whatever reason.
Also I've lost track: your engine bleed is at the rear of the intake manifold, but where now is the temperature sender?
Oh, and I just remembered something: be sure to do all this bleeding with the heater valve open, and make sure you are getting heat from the heater, otherwise
it could be trapping some air.