[39 Deluxe]

Agreed, crankcase will not be vacuum, especially considering how leaktight most of our flatties are.

PCV uses vacuum in the intake tract to vent positive pressure from the crankcase and direct it to the combustion chamber. PCV valve only opens at high vacuum conditions (i.e. idling, decel). It does not create a vacuum "leak" per se, as it is regulated by the design of the PCV valve itself. My real world experience is that the PCV system does not have a significant effect on manifold vacuum at idle, nor does it seem to have much affect on idle speed or mixture. I guess if you've got a ton of blow-by, results may vary.

PCV on a flathead, good idea to eliminate draft tube "steam" and oily vapor smells. I wouldn't expect any other improvements.

Also related to this setup....what about sealing the oil filler tube with a non-breathing cap? Or at least a cap that routes vapors to the air filter somehow...otherwise you've still got oil vapor escaping to the car. This would be similar to how Ford did it in the late 60's to 70's.