And to get back to the question: When is hot too hot? I would say when you boil...
One issue with comparing temps with an IR thermometer is emissivity. Basically this is the ammount of IR that a particular surface radiates at a particular temperature. For consistent results, it is best to measure the same type surface at all your test points. I have run into this when comparing readings across different types of metal pipes, such as copper to steel or stainless If you get a small spray can of high temp flat black spray paint, such as is used to touch up barbeques, you can shoot a small spot at each test location. This will help with consistency between readings. A small piece of masking tape at each location will also work, but the masking tape will eventually cook off of the exhaust. It is best IMO to be a matte or low reflective surface/color.
Your measured temps seem a little cold to me.
Under sustained load, that spot on the side of the cylinder head on my 6/1(on handcrank/fuelpump side) as measured with my IR runs around 200F
The water outlet pipe next tothe outlet/thermostat flange runs between 195-200F with the 195F thermostat I have installed.
Water return at the bottom of the cylinder measures between 70-90F depending on the temp returning to the heatexchanger in the secondary loop. The water leaving the heatex secondary loop has a thermostat that opens to regulate it's flow to achieve 120F.
The exhaust pipe right where it is screwed into the exhaust flange measurs 588F. That is after 1 1/2 hours at 3100W of electrical load on the generator.
140F out of the water outlet is WAY too cold, especially with a 176F thermostat. That is one of the largest drawbacks with that large tank. It takes a long time to get it up to temp.
How big/how many GPM is that pump you are using? I am guessing by the looks of it, it is way too big. Without a throttle or bypass on that pump to control it's output, it is cramming in large gulps of cold water from the tank bottom every time the thermostat gets hot enough to open. IF you had a thermometer in the head itself, you would probably see it cycle up and down quite a bit. You may even be able to see this a little with the IR, but the process of conducting thru the cast iron averages out the temps a bit. Probably the easiest place to see it would be the water outlet as the thermostat opens and closes. Temp swings are hard on things, and cause excess wear and tear.
Ideally, the engine would warm and the thermostat would open and allow flow. The flow would be slow enough that the water entering at the bottom could warm to a point by the time it reaches the thermostat that the thermostat never closes. Then all the thermostat would do is change the size of the opening to allow more or less flow and maintain a consistent temp based on the engine load. The 6/1 must dissipate around 17,000 BTU/HR of heat thru the cooling system at full load. That is around 283 BTU/MIN. With the 130F difference between inlet and outlet temps above, my 6/1 must move around 2.2LB of water per minute to transfer 283 BTU/MIN of heat out of the engine. 2.2LB/MIN is right around 1/4 GPM.
Heat in the combustion chamber is your friend. You need it for the injector so the vegoil sprays as clean as possible. You need it to maintain the combustion temp at a high enough level so the fuel burns cleanly and deposits less carbon in the cylinder. As mentioned, a 90C/194F thermostat would be a better choice.
A quick check of combustion cleanlieness is to pull the injector out and see how much carbon is baked onto the tip. You can also look down the injector hole and see a little of the piston top and IDI chamber.
