Check the baseplate sealing. Sometimes carb linkage will hit manifolds, especially pass side, and it can be very slight. Make sure there is no interference.
Huge idle speed drops like that are likely timing related. Another clue is run on after the key is shut off. Not enough initial timing.
Do the idle mixture screws make any difference in how the car runs? If not, what I would do. Set idle mixture screws to 1.5 turns out from bottomed.
DO NOT DRIVE THE CAR AFTER DOING EITHER OF THESE UNTIL TOTAL TIMING IS SORTED OUT.
Advance timing until the starter kicks back or strain really hard, then back it off a smidge. Start car and see if you can get idle speed to a reasonable range 800-1000. Adjust idle mix screws and speed. See if it still lugs down. Cams like the 509 are a PIA for RPM dropping because of the low vacuum to start with, especially with stock converters. See where idle timing is for the point just short of starter kickback. This is about where the idle timing should be.
OR
Start the car, vac adv disconnected/plugged, and sneak up on setting timing to 36*. DO NOT DRIVE THE CAR. If the engine races to a really high rpm turn the idle speed screw out as you feed timing in. Try to keep idle about the same as it is now.
Once at 36*, start slowly turning the idle set screw out lowering idle speed. See if you can get the idle down below 1000 rpm. If you turn it out and the engine makes a sudden drop in rpm, check timing.
Still at the original setting? If not, it dropped out some mechanical timing. Reset it to original 36* and continue turning idle speed screw.
If you can get it to idle in a decent range 800-1000, the issue likely is timing related. Check that the you have control of the idle mixture screws.
This whole test should take no more than about 5 minutes.
If you don't know the cam in your car, that's another issue. A good baseline idle timing for most mopars is in the 14-18 range depending on engine. More camshaft = more idle timing.
Set timing at 16-18, if it's a raspy cam, 20-26, and screw idle speed screw in/out until you hit the new found idle speed 800-1000.
