Originally Posted by
DMCMW Dave
Root cause on this is typically someone forgetting to change the oil for, oh, say, decades, OR running the car with a stuck wide open fuel distributor, flooding the oil sump with gasoline, turning the oil into about 0W0 weight. This in turn wipes a cam lobe or two. How did the insides of the engine look? If it looks gunked up that's the oil change issue, if it's remarkably clean then it's the fuel dilution theory.
If you find a bad cam, since that car is carbureted, I'd bet money that's what happened before they did the carb conversion.
Check all the rocker arms.