cooling not so cool

Chances are the radiator got painted up nice on the outside, but has 40 years worth of scaling and corrosion in the core. You will even see simetimes where they will still flow, but the buildup acts as an insulator and you coolant passes right on through the core without dissipating enough heat. Symptom will be slow increase in temp until overheating under long load, such as highway driving. My local radiator shop pulled the tanks and rodded the core for $80. Based on what you are describing, I believe rodding out your core will solve your issues.