Technical What speed is to hard on my 1959 Custom 300, 223

Yup, my Dad, bought 1 new Wedgewood Blue, same drivetrain you have, bought it new, for $2700 CDN.
Drove the car till 1967, had a 160000 plus miles, but it started knocking, so H e gave it to my Moms younger brother, he drove it like that till the engine seized one day.
He drove this car on the highways at 60-70 mph all the time, why he racked up a lot of miles going to see the old folks on the farm every weekend... but having said this, 56 year old engine, versus new or rebuilt, why not err on the side of caution and take it easy, especially on take off, and keep an eye on temp, oil pressure etc..