I have a friend who very recently got a 70s model van and it did the same thing. He put valve seals in it and cleared it up and it's one of the smoothest running small blocks I've ever seen. The original valve seals were broken all to pieces.
It would be great if that was the problem with this one. I think I will still need to pull the heads instead of fighting with them in the chassis. IIRC there's not much room under the cover either.
