340 camshaft question, yea another one.

Have you verified the TDC mark on your balancer?
Have you verified the action of your Vcan?

I know what you are trying to do, and I believe you are on the right track, but that cam should not need any help from the Vcan, nor much if any holes in the plates.

My current cam (276/286/110) is just a tiny bit bigger than yours, and it has been running on 14* idle-timing for years. With no help from the Vcan and very little bypass air. It idles at 750 easy, and pulls down a lot further.
If your port exposure is square, and your mixture screws are 1.5 turns out, and your float level is correctly set,and your idle-timing is 21*with the Vcan, then the high idle speed is caused by a combination of the bypass-air and excessive timing.
The tight TC is loading up the crank on engagement, and causing a sudden drop in airspeed and the T-ports cannot deliver the correct amount of fuel, at the reduced air speed.And so the engine falters.
When it falters the Vcan drops out and you have a downward spiral to a stall.
I only see one cure; eliminate the Vcan, increase the transfer exposure, and begin reducing bypass air, until the idle speed comes in smooth at 750ish.
Now a looser TC will get you through this tuff spot, and you might think it's cured. And I suppose in a way, it will be. But the loose TC is an added expense, and it will affect your fuel mileage, and I wouldn't be able to stand it that the underlying problem is still there.Besides I dislike an 850 idle where it doesn't need to be that way.
But first you gotta know where TDC is. Exactly.