Are you sure you're not just over filling it? Tractors don't have an overflow tank like a car. The coolant will come up 2"-3" in the rad when it gets hot.
Before tearing the head off, take the rad cap off. Let the coolant down to the top of the cores. Now run it till hot with the cap off. Have a look in with it running. See how high up the colant is. At this point, if you see bubbles in the coolant you know you have a gasket/head problem.