The idle quality now is much rougher. It's not going to like lowering it.
That’s amazingly high vacuum in gear for that combo.
My 383, with slightly more cam was about 7-8” in neutral and 4-5” in gear.
I’d probably start by just lowering the idle speed a bit and seeing how it behaved.
Leaner idle mixtures often run “smoother” and quieter...... but the idle in gear isn’t as strong(the motor won’t stay running in gear and the engine speed just starts to fall off).
My good reworked 850DP sounds like you added 10* of camshaft duration compared to using most normal carbs(at idle and low speed part throttle)....... but it actually makes the motor idle stronger in low vacuum situations.
I would say your idle vacuum is unusually high for a typical Stocker(I have no idea why that is).
Going to a richer idle circuit is a crutch to provide adequate idle fuel when the signal(vacuum) is quite low.
13” is more in line with what I see in a typical hot street/strip car.
