cooling at high rpm ?

Reminiscing - 1979 the new wife and I off to our honeymoon and her 68 Javelin is overheating. Mind is of course on other things. Pulled over and they had decorated the car, including a "Just Married" sign in front of the grill.

The high flow thermostat would have more effect at highway speed. Though as others have stated, don't think there is a problem unless as stated, your timing is retarded at cruising speeds causing more heat.

Buzzman said it right - thermostat sets MINIMUM operating temperature and has nothing to do with the max! Not a thing.  The only way a stat causes overheating is if it's BAD. These should be running about a 190 or so stat. MINIMUM, not maximum, temp is set by the thermostat. 

Everyone talks complicated stuff - don't worry about mixture, timing, etc. until you cover the BASICS. and unfortunately everyone thinks AIR - that's ONE flow to watch for but seldom is the cause in a car that's mostly stock because remember, these did fine in the 60s and 70s but this is 50 years later - why don't people think of AGE?
Air - fine, you do need air flow, but you need COOLANT flow and if your radiator is over 30 years old it's almost negligence to not consider it's probably pretty gunky at the bottom half. 
THREE of my AMC cars heat issues were 100% totally resolved with CLEAN radiators. My brother's 69 - with a clean radiator and a GOOD cap - his cap leaked pressure. Initially I simply put a new 14 pound cap on the car and it behaved a lot better, but again, new clean radiator resolved all other heating issues. 

I was really concerned first time I put a digital set of gauges in my vehicle - YIKES! 120! then I remembered - 50/50 mix plus pressure and it won't boil until OVER 260! 
My truck ran 230 much of the way back from Colorado Springs last month. Another guy I know has one and his ran 240 pulling a trailer. That's the top side for me - in my mind, but still not going to boil. 

AMC did testing and I have a TSB on it somewhere - showing the different flow rates of pure water vs 50-50 mix water and coolant vs. pure coolant. 
The most, the highest flow rates by gallons per minute went to the 50/50 mix. Their point - besides protecting the water pump and preventing cavitation of the timing cover, the mix cause a higher coolant flow, and better cooling. 
But that is negated with a 40 year old radiator. 
Anyone running a 40 year old radiator and thinking "I looked and it seems clean" is not seeing the bus about to hit them.  Get it professionally cleaned - like I did. 
Original cooling systems, no fancy fans, no fancy this or that and my cars run cool, including my 73 377 HP 360 that's been abused. Factory radiator factory shroud, etc. - it's just fine.

A lot of good things suggested to check out.

I'm on the "get a fan shroud" side.

Also, how is the fan operated....direct/solid or via a fan clutch?  If its a fan clutch maybe it isn't working properly.  Just another thing to check.