GM stuff is full of lectrovalves for engine vac that should work fine. Get one that's NC and opens on power, and tap the compressor clutch circuit for the power. Tune the idle up air amount with the aquarium needle valve. Plumb it from front to back of TB. Air cleaner case to a ported vac tap should work fine. May have a minor lean out effect on a/c'ed idle mixture, but you're rich at idle, anyway, right?