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'70 Charger Overheats after 40 minutes on highway today [but before this it did not overheat at idle or shorter trips]

When I drove my 70 Road Runner across country in 2015 we drove sustained speeds of 90+ MPH through Kansas with a top speed of 108 MPH. Our 90+ speeds lasted about 2 hours or so. There is no reason, from a cooling system perspective, that your car shouldn't be able to do the same.
I totally agree. I never heard of a well running car overheating due to a carb not putting out enough fuel. Back in the day, I used to drive my '71 Pontiac for around a 600 mile trip.
Well some good news. In addition to cleaning out the system and replacing the thermostat, a mechanic installed the new radiator. I drove it back 35 miles for 60 to 70 MPH and was pegged at around 180 to 190F for 99.99% of the trip. For one split second the temp jumped to 205For so and then fell back down to 180 to 190F. I don't know if that temp was legit or my temp sensor is doing something funky. But now I have a flushed system with a nice new large radiator and new thermostat. Thank you all for your help! Don't want to jinx it but so far so good!
I have found that in a properly filled cooling system results in a steady needle. When there are pockets of air or a partially filled system in play, the gauge needle seems to rise sharply sometimes. I suspect that is when the sending unit is exposed to steam rather than the coolant.