I'm not sure what kind of COE you have, but we drove 60 back in the 50's all the time. There is a lot of fan noise, valve train noise and wind noise, but it doesn't hurt them. Just make sure you have a good radiator to keep it cool. If you calculate the rpm, your hp is probably rated at 55 or 60. I think if someone would develop a fan clutch for these old things, people wouldn't scared to drive them at highway speeds.
