Mopar 383 help please

Are you sure you are on a manifold vacuum port and not a ported vacuum?
Does it have power brakes? Possible leaking booster. Also check the hose going to it. Check the pcv valve and make sure it isn't stuck open. Check the pcv hose, I have seen them collapse and suck air.

Did this just happen all of a sudden?
It runs fine other wise?
At 5" of vacuum I would expect the car to die just trying to put it in gear.
At what rpm is it idling at?
Throttle may be opened to compensate for a vacuum leak, thus getting no response adjusting the mixture screws.