My guess is the butterflies are sitting too far up the transfer slots. Fix that first.
How much rpm do you loose going into gear?
What is the Neutral/Park idle speed ?
Idle timing is?
If that cam is really in at 105, then is it a 110 LSA (Lobe Separation Angle)?
Idle timing must be set with the Vcan defeated. And the Vcan should be plumbed to the spark-port. Power timing should not exceed 36*, 34* is safer with iron heads; until you are ready to tune that end.
The point is; IMO, your neutral vacuum is too high compared to your in-gear vacuum, and the in-gear vacuum is too low for that cam. Both are signs of a faulty T-slot sync, usually due to wrong idle timing, But , I agree that the PCV may be a little large also, for that cam. Possibly. But I think it's probably all in the Sync.
Set your transfer slot exposure under the butterflies to about square, or a tiny bit shorter than wide.
That cam, if a 110LSA or more, should idle nicely at 550 in gear, 600/650 in neutral, once the sync is set and the idle-timing adjusted to achieve the above idle speed. At that time, if decent idle quality cannot be achieved with the idle mixture screws near the center of their adjustment range; THAT is the time to consider the PCV.
Hang on, I know nothing about the 390 Holley,except that it has extremely small venturies,lol.
Ok so that is where I'd start.