Hot at highway speeds

Had a 69 Roadrunner that did the same thing, highway driving , and it would run real hot. In town driving , no problem. Took the radiator out to the radiator shop and they cleaned it out, and no problems after that. That car went on to be the Mopar Nationals Grand Prize give a way car in 1992, and I think it is on ebay right now for $42,500.