Interesting thing about those factory Mopar temp gauges (and add the oil pressure gauges as well) is that back in the early 60s, they seemed to be pretty accurate, but by the end of the 60s and into the early 70s, that accuracy went right out the window. Those nationally imposed emission standards of 1968 almost instantly required higher motor operating temps to meet them. That was about the point all the manufactures dumbed down the factory gauge accuracy. Too many people were complaining about the cars running hot, the easy fix for them was alter the temp gauge readings. The reason the higher operating temps are upon us is because of the ever increasing emission standards up to the point the operating temps started effecting the life of the motors. That was the point better designed motors became a reality, Those better designed motors started showing up about the 2000 model year vehicles. The current crop of new motors are design controlled by bean counters trying to save a few cents on everything they can. Save a nickel on every car you produce may not sound like much until you build a few million vehicles, that is a lot of nickels.

The old designed motors seldom reached 200 degrees, remember water that is not under pressure boils at 212 degrees, add cooling system pressure and that boiling point can get up near that 260 degree mark. Detroit started pressurizing the cooling systems in the late 1940s. Thank the old Ford flat head V8s for that. Any old motor built after about 67 has better materials to withstand the higher temps the manufacturers knew was coming. Unless the motors that are older then 67 have problems, it would be a challenge to get them up much over 200 degrees with a pressurized system. Those motors and cooling systems were designed to keep the motors cooler then anything built after 67 or so. Those high performance big block motors of the late 60s caused a lot of temporary challenges for the car companies, at a time they were trying to raise the coolant temps to meet emissions. Suddenly big motors with increasing power, put into smaller packages with air flow restrictions, hit them all at once. That pretty much describes the late 60s and early 70s.