Not sure where to get one at, other then the few that are in a draw in my garage. But Ford escort deisels used a vac relay. They turn vac on when you put power to it. One place that may be able to get such things would be that one locker company down under. They make vac lockers. Not sure if they are even around anymore.
A qtrac switch would work, as would any of the Selectrac switches from the 80's FSJ's (same mechanicals as the QT switch, but with a lever rather than a knob).
My '70 Cutlass used a Transmission Controlled Spark device, which cut vacuum to the distributor except when the tranny was in high gear. It used a solenoid that went in-line to the distributor, and cut vacuum when it was powered (or not powered; I forget which way it went).
In any case, there are plenty of emissions vacuum control valves that you can adapt to that use.
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