Just my two cents after not reading everything. I can attest that my 350 Olds diesel had the standard at that time 185 degree thermostat. No different than a gasser. Whenever I ordered a car, I always added the heavy duty cooling option. It mainly was a larger radiator and a larger mechanical fan or in the case of electric, you got two fans instead of one. Also on the diesel and sometimes the other cars, in severe winter temps, I would have to add cardboard in front of the radiator in order to get it to operating temps. Especially the diesel. I don’t know what the flow was but in general, a larger radiator was a good option, especially as the car and radiator aged. An extra margin of cooling.
When replacing thermostats, I always did the water pan test first and usually used Stant brand. Now I’ve started just buying OEM for about 3 or 4 times the cost, but seems to be better quality and more reliable. If you are having problems go with a quality thermostat and cover the front in north country winter temps.