You say no idle mixture screws in end carbs yet your photo shows that front carb appears to have them,,,,though not factory carbs, they too have them as is stock from mopar,,,just located differently and factory plugged.

2 full turns out on mixture screws on center carb still sound suspiciously excessive, perhaps masking a vacuum leak or transition butterfly positioning issue. If you do have mixture screws in end carbs, how far out are they set?

Try throwing the car in neutral say at 50 mph. While coasting note where tach is registering. Should be your normal idle speed. Hit the brakes about as heavily as when you would when coming to a normal stop.

What does tach do then? If remains constant, would suggest that fuel slosh is not the issue. But could suggest again that transmission or convertor is causing a temporary drag on engine when coming to a normal somewhat abrupt stop.