Check the pan on the engine. They used to use nylon coated timing gears for a "quite chain". My 71 barracuda with a 383-2bbl had timing that was all over the map.

Pulled the pan and found it full of nylon chips. Pulled the timing chain cover and the nylon gear blank had razor sharp teeth as the nylon coating was gone. Substantial timing back lash.

Cleaned the motor best I could, put in a TRW double roller timing chain and sprocket set (all metal) and kept on going.