Slant 6 Operating Temp

I always test T-stat's in a pot of water on the stove first, checking opening against a glass bulb thermometer and IR gun. BTW, the IR gun compares very well even shooting thru water, but you must understand that the viewed area is larger than the laser dot. I found a M-B T-stat opened slightly early and too slowly, compared to a new one, and swapping it made it run at normal 82 C vs 60 C before.

Once you verify the T-stat, you are correct that you can count on it regulating correctly, so the dash gage display is then "merely interesting". All my Mopar's 1964 to 2002 have just tics on the dash gage, no numbers.

I always install a 195 F T-stat. A lower one won't help in avoiding overheating. Once the T-stat is full-open, it can't do any more than that.

Also, understand that a T-stat doesn't regulate an exact temperature. At higher heat load, it must open more, which requires a slightly higher temp than the initial cracking temp. In engineering, this is termed "proportional control droop".