How to measure engine temperature?

Both gauges of the instrument cluster of my Ford 5000, 1965 - fuel tank and water temperature - are off. At first, the needles were showing half of the readings: the fuel was showing 1/2 even when the tank was full, and the temperature gauge was hardly moving at all. But after I've replaced the voltage regulator, both gauges seem to overshoot: the temperature gauge is in red, when the engine is hot. I'm thinking of lowering the input voltage by adding a rheostat (or just a load resistors) after the voltage regulator. That's not the issue though.

The question is how can I tell that the engine is at its proper operating temperature at which point the needle should be in the middle of the gauge?

I read somewhere that diesel engines operate at about 200F. But when I touch the radiator it's not scolding hot; it's hot, but it's quite bearable to touch, which means it's probably around 140F. I know, the temperature outside of the water tank is lower than inside, but still. If I need to decide that the engine is at it's proper temperature - how would I do that? Is there a specific point on the outside of the engine, which I could measure with an IR gun + some kind of correction formula to take the temperature drop into consideration?

This post was edited by jrslv on 10/23/2021 at 08:48 am.

If both gauges are reading high you have a faulty voltage stabilizer.

You could point an IR gun at the upper end of the engine block and come up with a fairly close number. Myself, if I wanted to know exactly
what the temp was I'd plumb in a mechanical gauge. To do that on a 5000 you'll need to drill and tap the intake manifold for 3/8 NPT thread.
Once you pull the manifold off you'll see exactly where you'll need to drill the hole.