Did you ever get that interment overheating problem fixed? Another thread here on the group has jogged my rusty memory. I had a '59 Ford back when I was a kid that would do the same thing. It would overheat at highway speed but was fine around town or at idle. I did everything my friends or I could think of without fixing it. A friend's father suggested replacing the bottom radiator hose, that fixed it.
Seems like there was a flap of rubber that had broken away from the inner wall and high water volume/speed would force the flap up and partly block the hose.
If that isn't the problem, don't send me the bill. If it is, you can send me a check. hehehe
Jack