Mildly modified 318 PCV valve selection

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.