Not an original-equipment fix, but how about valving the vacuum with a fuel tank selector valve? It could be dash-mounted and plumbed to apply or remove the vacuum from the axle-mounted shift system. It might be necessary to create a small bleeder orfice somewhere to prevent vacuum from being trapped in a line when it's supposed to dissipate.
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!