It would seem that 2 gaskets, one above and one below the valley pan, would potentially raise the manifold enough for alignment issues with the intake bolts.
I have had some issues in the past where the misalignment allowed valley oil to get pulled into the combustion chambers through a gap between the intake and head.
In that case, with the addition of gaskets, the valley pan ribs couldn’t crush, as was intended for sealing.
Regarding the service action for poor manifold machining, those gaskets were much thinner than the gaskets normally made available today.
