2bbl to 4bbl conversion?

If you get the transfers synced up and running with a reduced idle-timing, and the engine has no tip-in sag, then you can reduce the fuel from the idle mixture screws.
You gotta keep in mind that the transfers are your primary low-speed fuel suppliers, and the mixture screws are just idle trimmers. So if you double up the transfer fuel, and the AFR goes really fat, the you're gonna have to figure out how to burn all that fuel,lol... or how to lean that circuit out. It's not easily adjustable so you are gonna have to re-engineer it. You're gonna have to find the fuel jets, or just holes in this case which if I recall, are at the bottom of the removable clusters,and make 'em smaller. And you may have to fudge the air-bleeds at the top too. And perhaps trim the whole she-bang with fuel level.
I suppose on a single-plain intake you could stage the one carb to come in later, like you did before, which might make it easier to take off with, but I think there will still be some cylinders going lean. Maybe with a larger plenum spacer, but I'm just guessing.
With a dual plain I would think non-progressive would be just too jumpy no matter how lazy the timing was. I think I would just run a 2800 TC, and let the low speed go fat and let it get lost in the fluid coupling.
Get the idle right with all 4 transfers contributing first, and see where it takes you.
I think I would change the throttle ratio so it takes more pedal for a given throttle opening. That will make it easier to drive.
A long time ago, I wanted to do just what you are now doing, so I'm real interested to see how you make it work,
As for the two-stage timing curve, most smogger-teens, with electronic ignitions,that I have worked on had those from the factory. If you pop the timing plate up, you will see the two advance weights and their springs. You will see one spring with a long loop. Until the flyweight s are spun up to the limit of that loop, it does nothing. So both weights are working on the other spring until that point. Don't put a chevvy spring on there, cuz the timing will come in, in a real big hurry. Find a spring that is a lil lazy to start advancing, but hits in the neighborhood of 24 to 28 degrees by about 2800 to 3000. Then let that long looper slow the curve down to all in around 3400 to 3600. You may have to start with an initial timing of 8 or 10 to to get a slow enough idle speed with all 4 transfers flowing. I wouldn't worry about it cuz it's an automatic, and if it's idling it ain't pulling. And with a 2800TC it won't be pulling for a long time, and when it gets to 2800, well, hopefully you will have figured out how to get the 28* up there,lol. How the timing gets to TC stall would not be a big deal for me. Cuz when the teener hits 2800 with 28* , it will be a happy camper.
If you have a factory 1750 type stall convertor, your tune is gonna be a whole lot harder to hit.
If it's fat below stall, I would put some hotter plugs in there until you get it bugged out, or else it may start eating plugs.
Anyway, like I said, I've never done what you are doing, so this is all just Idle chit-chat.
Someone else may be along shortly and have a whole nuther story.
No matter what, Happy HotRodding.