Two things common on those engines

The oil passage to the rockers plug up in that short horizonal area where it goes sideways toward the bolt in the hollow rocker stand

Those engines had the rubber umbrella valve stem seals and they when they got old and brittle they would crumble up in zillions of rubber bits and eventually get down into the pan and plug the pickup screen---there generally is evidence along the edge of the head under the valve cover and/or you can see that the seals aren't on all the valvestems
These were one of the first engines to get fit with PC type seals on the engine

Jerry