I agree that it's possible to ruin a valve, but unless someone is beating a 70 year old engine to death auto-crossing it or some other foolishness, you're never going to damage the exhaust valves. High rev racing engines or engines operating at the redline all the time are in danger of burning valves.
Driving antique vehicles under normal circumstances at normal speeds and normal loads won't hurt them.
I've put a couple hundred thousand miles on The Bismarck over the last 25 years. That included highway trips of over 500 miles at 75+ mph as well as local stop and go driving. It has only ever had unleaded fuel in that time and the heads are stock.
The same is true with the other vehicles. Drive like a normal human being and you'll be fine.
