</font><font color="blue" class="small">( The tractor does within 10 to 15 minutes over heat at higher rpm, to the point that the radiator steams and must be shut down. )</font>
I have to agree that it sounds like a belt slipping since you say it overheats at higher RPM where the belt would have more opportunity to slip.
Try this: Apply pressure to the belt between pulleys with your thumb. You should get only 0.25 - 0.34 in. of deflection. If it's greater than that, loosen the alternator mounting bolts, place a lever (maybe a large screwdriver) between the alternator and the engine block pulling the alternator out until the belt is at the right tension. I would tighten it if it moves more than a 1/4 -1/2 inch myself. If that doesn't repair it, take the radiator to a shop and have it cleaned properly. You say it's low hour tractor, but you don't know how well it was maintained before. It's possible that the fins have a lot of debris deep in them. Also, I would drain the coolant and put it back at 40% antifeeze and water. You could try using compressed air, blowing from the engine side out to the front. Keep the PSI of air fairly low so as not to bend the fragile cooling fins and make sure you don't hit the fins with anything. I hope this helps. John