It could be a defective thermostat, or even improperly installed. I know there are some thermostat packages that convert to a newer automotive style thermostat. I put one in my 660 last year, but haven't worked it enough to put a demand on the cooling system. It took me a couple minutes of looking at the thing to figure out how to get it in there right. With the newer style thermostats having a smaller opening, I wonder if they cause a restriction of flow compared to the old style? 

If it were mine, with what you have told me, I would pull the thermostat, put it in a put on water and heat it on the stove with the meat thermometer in there. You should see it open as the water approaches 180 degrees, and it should open considerably once it gets over that temp. If you still have the old 160 thermostat, you could put that in the same pot and compare when they open, and by how much. You could also put that one back in the tractor and see how it works. I know it isn't fun tearing into a freshly painted tractor, but the meat thermometer has confirmed that it is heating up, and the only way to change things is to go in there.