Technically, 1964 was the first year for 50:1. However, I would run that engine somewhere around a 40:1 mix just to be safe. You will notice zero difference in smoke. Hope you have a new water pump impeller and have made all your checks if this is a new to you engine?
Two cycle oil technology has come so far since the early days. The lubrication quality with today’s oil at 50:1 is greater than what was available back then. That being said, what Pappy recommended won’t hurt and may give you piece of mind.