Sounds like the engine has done very good!!!! A 37 year old engine with less then 100,000 miles that is still in running condition is great!!! Could be a variety of things, at that age with those miles it obviously spent a lot of time just sitting. The older oils had a nasty habit of building up sludge throughout the engine and in your case it sounds like some of it has made it into the oil passages that feed the rocker arms. A clean out and new pump like Don suggested may remedy the problem. Or perhaps the lack of lube to the rockers has already done the damage and it's also going to require some repairs on the rocker arms... Lots of old parts in there that have been sitting for awhile, perhaps it's just time to freshen everything up?