my '57 235 used to do the same and I fought it with sealers and new gaskets, cork and synthetic. pulled the cover and checked for flatness and was surprised how bad it was from past sins from who knows when. When it had leaked in the past I think previous owners must have continued to tighten the screws until there was no way it would ever seal.

Larry
Old man᠁Old truck᠁neither one goes very fast. All you need in life is TIME, PATIENCE and MONEY.
If you are missing one component, you'll need an abundance of the others two.