Tom I have had the radiator off my SC twice and to a good radiator shop. They cleaned it up inside and out and replaced the overflow line and it still will not hold coolant above the top of the core.
I'm going to add a thermostat in the spring with Mike Schotts idea of the two holes in the flange. Like yours my SC barely moves the temp gauge. Maybe this will do the trick. My good friend says those tractors rum more efficiently up around 180 degrees anyway.