Another thing to check is the ignition timing. I got a call one time on a 40 Chevy car with the 216 that would get hot within a minute or two of starting and start pushing coolant.
Turns out the timing was quite a bit off, and was firing too far down in the cylinder. Reset the timing and no more heating issues.
If you followed the directions in the manual, then your head gasket is likely okay.
Make sure you leave about 1" of clearance from the coolant level to the radiator cap opening. Since there was no recovery bottle on those old systems, there needed to be some expansion room in the radiator itself. They will belch out excess coolant if over filled.
I have heard arguements both ways about running with and without a thermostat in place.
On the older rigs the radiator is usually quite capable of running without a thermostat, but in my humble opinion I think it better to have one in place. ( And that opinion may not hold water)
The newer vehicles have thinner radiators to save weight, and if the flow is too fast through the radiator then it is inefficient.
Let us know what you find.
