I’ve run a lot of old cars on regular gas with no problem. Although the oldest one I have is a Buick with aluminum heads, so it’s not an issue with that one. And I suppose the other one from the 60’s is a VW with an all-aluminum engine. However, I have owned a few pre-74 pickups with iron engines and they did just fine with no lead additives.
I’d talk to the owner about it-- chances are that most cars from that vintage that have been on the road for any length of time since the carter administration have had their valves redone to be compatible with unleaded gas. Unless the owner has been driving it extremely few miles or personally putting in lead additive since the mid-70’s, I wouldn’t worry about it.