The Nebraska tractor test machine ran at about 155-160 degrees in general use, and never exceeded 180 degrees. Their air temperatures were around 75 degrees. Knowing that, if I were running a 336 and saw temperatures over 200 degrees, I would assume my cooling system was somehow compromised. As long as the coolant is not boiling over it's not overheated, but knowing how effective the cooling system on that model is supposed to be, I would start to inspect things were I to see temperatures above what they recorded. The highest temperature they recorded was for the tractor at PTO speed and loaded down as heavily as the engine would sustain, for two straight hours. This is more demanding than most people would ever be on their tractor, so is a good marker for maximum temperatures. The link to the test for your YM336 is http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=2810&context=tractormuseumlit