I had a similar problem with a car. It turned out to be a plugged up catalytic converter. The faster I went, the harder it had to work to push the exhaust out, so it overheated. Around town it was fine because of the lower RPM.
BTW, I had a '79 Cutlass with the 260. I called it the "Gutless Supreme" because it had no power at all.
It was much better after I put a '73 350 Olds in it. It was a direct bolt-in, as is a 455.
