OK, yeah that transition slot looks ok.
Well, if the PCV valve also was connected and it still wants more air you could try this:
-Disconnect the hose from the pcv, install a fitting with a small ball valve on the hose end and make sure it is leak free.
-Run the engine and gradually open the valve and observe engine rpm until you get your wanted idle speed.
-Leave the valve at that position!
Now you can start fingering a bit on the rest again, try to reduce your initial ign. advance to 16* or so.
RPM will likely drop, try first with allowing a little more air in by the ball valve. If that increases the rpm it means the fueling is still sufficient to support that amount of air, if not open the mix. screws 1/8-1/4 more and see if you get a nice steady idle.
This way you can play a bit with it to reach your target rpm.
If you did not adjust the idle screw and the transition slot is still square your carb is set good, mixture screws should be around 3/4 - 1-1/2 turn open.
Now the only thing remaining is your by-pass air feed.
After setting that valve do not change it, make sure it stays in the same position.
Remove the valve/fitting assembly and hook the hose back up to the PCV valve, start the engine again and see what it does.
Quite sure it will not run as it did with the ball valve setup,
In this case it means your engine is, as you said already, not getting enough air.
To solve that there are multiple solutions:
-Open the secondaries. Now you tried this already and it did not work out. I never tried this to solve my idle issues, but a great help of someone with more knowledge than me told me not to even try it.
The reason is that the air that passes the secondaries is leaning out the rear cylinders A/F ratio, so your forward cylinder will run rich (because you compensate with the mix. screws) and the rears are lean.
-Drill holes in the primary throttle plates. I did try this and it does introduce extra air. The benefit of this over opening the secondaries is that this by-pass air is mixed directly with the normal air/fuel mixture and stabilizes the A/F to equal over all cylinders.
You hook up the PCV as normal, drill 1 hole in each throttle plate, start small (1/32 or 3/64 drill bit) and increase the hole size until you got that same good idle quality and rpm.
Drill the hole near the middle of the plate, on the same side of the shaft as the transition slot (forward) and make a very small chamfer on the hole.
Like below example.
You should have 1-9/16" throttle plates, Holley/QF sells sets of new throttle plates for around $15 so if you end up with too big holes you have an easy solution of restoring it to normal.
When replacing throttle plates strictly follow the instructions, its not hard but needs to be done right.
-Get an M/E Wagner PCV valve. You might have heard of them, this is an adjustable PCV valve which allows you to tailor the air feed at idle to get that right. And also allows you to adjust the point where it will open another valve to increase the air flow at cruising and WOT to make sure your crank is getting properly ventilated. This is the most expensive option though.