Large RPM and Vacuum drop when shifting from park to gear

Put that vacuum advance line back on the sparkport.
Make sure the secondaries are closed up tight,but not sticking.
I'm assuming the float level is according to design.
Increase the idle timing to 10/12.
Idle the engine down to 650 in gear,
and it should be fine.
If it's not;then
Now, let the tuning begin

FORGET tuning the idle to the highest vacuum; it DOES NOT WORK. You just proved it. Leave the gauge in the toolbox.

By tuning to the highest vacuum, with the Vcan on the manifold, you cranked a bunch mechanical idle-timing out of it, to get the rpm down. But you could not get the rpm to go any lower because the butterflies were shutting OFF the transfers and it wanted to stall.
Obviously to me, the cure is less advance. By ditching the full-time Vcan, you will have to restore the mechanical timing, which will allow you to increase the throttle opening, to restore the transfer fuel ratio, back to normal. Now your engine will have enough power to not stall going into gear.

The amount of IDLE-timing you can run is governed by;
where the butterflies like to be up on the transfers, and
The rpm that you target, to idle at.
A stock cam might idle at 500 in gear. Each bigger cam might like 50 rpm more. By me saying to target 650 in gear, I am guessing that your cam is ~ 3 sizes bigger than stock or 212 to 218 @.050, IMO, a safe guess. It is my hope that this guess will put your transfer slot exposure underneath the butterflies to about square to perhaps a lil taller than wide. If you want to, you can just take the carb off and look, and set it there, but if you do, then LEAVE it there. Do not set your idle speed with the speed screw; instead, use timing. Set it to 700rpm with timing, and then put it into gear and fine tune the rpm with timing. If it bangs into gear use less timing. If it wants to stall, first twiddle the mixture screw a bit richer. If still wants to stall, add timing.

After you get really close,
then you can try up to a turn more or less on the speed screw, but keep track of where you are at all times, relative to the starting point, so that, if the need arises, you can get back to it, without taking the carb off again.

You will know when the butterflies are too far down the transfers by the hesitation that shows up during a slow throttle tip-in . Too rich is harder to feel; so I always go leaner until it stumbles, then back up.

Good luck