My 70 gets hot at highway speeds if the outside temp gets above 80+ degrees.
What I found out was I am missing some of the foam strips that surround the radiator and the bottom half of the shroud.
The way I understand it is without all foam seals and of course the bottom half the shroud the car doesnt force enough air through the radiator to cool it at speed. I guess it does make sense.

That would be where I would look first. (Its hard to know what you havent got!)

If you have a front liscense plate try  removing it and see if it makes any difference. Cured mine.

Hey Guy's thanks for the info. Well what I found out is the fan clutch is not working right. I ran the engine for awhile and found that I could move the fan. Should it lock up when the engine heats up. The hose does not have  a spring in it. Called Napa to ask if the hose I bought should have a spring and he said no? Told me to try a hardware store for a spring I know the hose I took out had one in it. All the seals seem to be good but it is missing the lower dam. I will get a after market fan and look for the right lower hose.

I didn't reinstall the spring in my bottom hose, I was told the newer hoses do not need the spring as they are less likely to collapse. No problems since install and a good 800 miles later. I did save the spring though.
 

Installing the missing lower dam will most likely take care of your problems.

Ok time for a update. I put a flex fan on and my over heating has gone away. The only problem with it is its noisier but I did get the over heating solved.

Great news, heres my story, Ive tryed to run flex fans on both of my cars, on the 69 I had side pipes and you could barely hear the fan, I drove the that lleh out of that car, I wanted another car so I put it up for sale, I take  a  propsect for a ride, I hammer it and I pick up a vibration, I get home lift the hood and a blade is sitting on top of the intake half under the air cleaner..no damage the guy buys the car, fast forward to the 72, the car doesn.t overheat but when I turn on the AC the temp would go to 220, so I try another flex fan, but this time I watch the fan, after about 3- 4 months I start to see stress cracks starting to appear on the SS blades where they attach to the hub, put the stock one back on, I didn't buy cheap on both occasions they were the highest rated Flex-a-lite's with the blue hub, maybe its the way I drive but I think its the constant flexing of the blades,be aware and good luck.

Yes good to know thanks.