Weird temp issue

[Chevy nutcase]

Had my 66 out today after thoroughly flushing the system and adding an overflow tank to eliminate trapped air and I still have a somewhat high temp issue that's been pestering me since I got my temp gage working. It's a bone stock 250 with what I'm assuming is an original Harrison radiator and the odd thing is the temp is fine putting around in traffic but as soon as i get on a stretch where I can maintain 50+ mph the gage creeps to nearly pegging the temp gage. I used a laser thermometer on the engine water neck and get an acceptable 185° when the gage is in the middle where I like it. When it tops out it's reading 200°- 210°. The only things I've done to the cooling system since I've owned it was adding a clutch fan, changing the thermostat to a 165° when I completely stripped the motor to clean and paint it and in doing so had the water pump off and thought it was fine and of course flushing it with the type you leave in and drive around for a few miles. I read on other posts about adding a front air dam because at higher speeds a low pressure bubble forms in the engine bay not allowing air in through the radiator. I will probably do this regardless but the truck has existed for nearly 60 years without one. Love to hear your thoughts.