'66 Valiant Running Hotter than Expected

I'd be wondering what actual temperature it's getting up to. Factory gauges really don't tell you much, and aren't that accurate anyway. I run a mechanical temperature gauge in everything for that reason. Find out what it's really doing, then go from there. My very amateur experience is that if the car is running hot at interstate speeds, it has a coolant flow issue somewhere. I'd get the radiator flow checked, then check the thermostat to see if it's opening when it's supposed to and as much as it's supposed to. That said, I have two nearly identical small blocks, one a 340, the other a 360. Same cam, same intake, same carb, same exhaust and same heads. Both have three core factory type radiators. The 340 does have a bit higher compression ratio, 360 of course has 20 more cubic inches. The 340 never gets over 180. The 360 when fully warmed up on a hot day likes to run at 210. Never have been able to figure out why, but new cars routinely run that hot, so I have just lived with it for 30 years.