How do you know the coolant temperature? My 64 & 65 cars just have arbitrary marks on the dash gage, and I recall that in my 69 Dart. Regardless, use another measurement to verify. I use an IR gun which always compares well with the the dash gage in my M-B's (have numerical marks). I use a 195 F T-stat in my Mopars for better mileage and less engine wear, but many opinions on that.
Twice I changed T-stats and that did fix a problem. My 96 Voyager was showing too cool on the dash gage in the winter, and increased w/ cardboard in front of the radiator. I found the factory T-stat in pieces w/ spring tower off (many cheap ones today). My 1984 M-B was showing 60 C on the dash whereas 80 C is normal, and IR gun also showed 59 C by sensor. Tested T-stat in hot water and it seemed to open & close normally, and looked fine. Repeated against a new T-stat and some Mopar ones, w/ thermometer and IR gun. The IR gun could read the T-stat under water and agreed well w/ the water temperature. I found the suspect T-stat opened too early and had a sluggish response w/ temperature. Installed the new one and dash gage then showed 82 C. Probably improved the mileage. I thought that was a clever test, so posted on a M-B site and one guy said "of course one always changes the T-stat if any question about your coolant system". A bunch of nancy boys there who never work on their own car. You guys are more resourceful.