Thanks for the feedback from everyone.
Regarding the timing, I've read in several places that retarded timing can cause overheating. The belt tension is good and the belt is the correct size. The water pump doesn't make noise, though I've not taken it off to see if the impeller is in good shape.
The thing I haven't figured out is why did the tractor run for close to an hour in extreme heat before boiling over and then after flushing the radiator and engine and then installing a thermostat and new coolant it started pushing out coolant within 5-7 minutes. And I wasn't able to put in more than 2 Gallons of coolant. something changed because of what I did and I'm stumped. The thermostat is facing the right direction. Is it possible it's just a bad thermostat? I might pull it out and run it to see if that changes things.
No doubt the radiator probably needs to be worked over and probably the ports in the engine need to be cleaned out. I'm learning that I should have done my homework better in this tractor before I bought it. :)