Was all ok before installation of six pack system, or did issue occur after upgrade? Knowing would help eliminate other possibilities like timing, fuel slosh etc.

Idle mixture screws out 2 full turns sounds quite excessive,,,,possible vacuum leak somewhere?? Where are end carbs set. Have you checked butterfly location in carbs to ensure that you are not in transition circuit at idle? If so, all bets on proper idle performance could be off.

What is engine vacuum at idle?

Are you certain that power valve gasket is properly positioned,,easy to miss this,,however 2 turns out on mixture screws, I would tend to dismiss this idea, but should be checked if all else fails.

Have you tried throwing transmission into neutral before slowing down and stopping? If not try that. What happens to idle then? Could rule out fuel slosh.

Is it possible with torque flight that transmission not shifting down properly or that torque convertor is placing some temporary drag on engine. Again placing in neutral before slowing could provide troubleshooting info.

My 300b with torque flight can be a bit flaky at times when stopping car,,,also multi-carb(4 barrels) engine.