Original equipment type stovebolt distributor housings that move when the vacuum advance operates "mark their territory"- - - - -and there's virtually nothing that can be done to prevent it. Installing a PCV system instead of a road draft tube keeps a slight vacuum on the crankcase, and will minimize the leak, bit it might not totally eliminate it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!