Ed, It appears I gave you BAD info on the purpose of the ported vacuum switch. It is part of the pollution control system used on some models. My '73 340 does not have A/C but there is a plug in the intake manifold just behind a heater hose pipe & beside the thermostat housing on the passenger side where illustrations in my service manual show it would go. Plumbing is bottom to manifold vacuum which I believe comes from the pipe that supplies the power brake booster, middle to the distributor advance, top to OSAC valve which is mounted on the passenger side firewall.
I have reviewed the manual and A/C vacuum source is never identified but it may have been a tee off the manifold vacuum or another port on the brake booster pipe. I say this because there is a drawing of a brake booster pipe with multiple ports.
Hopefully someone with an original A/C car can clarify this for you.
