If a PV is ruptured or open it will greatly affect idle even if its "idleing" in the idle curcuit, If the idle screws can be closed & it still idles its still getting fuel, You can rule out the needle valves because its holding fuel level, You say its getting 18 inches vacuum, thats very strong & the stock 6.5 PV should not be an issue. Things i would check.

Make sure you have the "right" primary metering block gasket, you said the carb ran fine before, so there shouldn't be any issues with the IFRs being tampered with.

With good vacuum like you have i'm sure the primary throttle is not opened up that much allowing it to get into the main curcuit.

I would double check the PV also, make sure the gasket is in there right. See if the carb is dripping fuel from the base when lifted from the manifold, i'm sure it is.

If for ANY reason the engine spit or kicked back through the carb it can/will take out the PV, especially if the base doesn't have the "blowout" protection check ball installed. BB mopars are notorious for doing this if timing isn't close or you get a lean pop.