I’ll just add this, as a reference on where you might need to end up with regards to ifr sizing.......

In my experience, when using cams similarly sized as what the OP is using...... if one had a normal Holley aftermarket performance carb that had the traditional 2 corner idle circuit....... like a 4777(650dp), or older model 4779(750dp, before they came with 4 corner idle), or a 3310(750vs), you would find that those carbs would have marginally acceptable idle circuit capacity(it wouldn’t be good enough to where I could leave it with the stock calibration).
And those carbs all had the fixed secondary idle discharge ports contributing to the fuel flow, in addition to having a more performance idle calibration(as opposed to the factory emissions calibration with a stock cam that provides nearly 20” of idle vacuum).

What I would do to solve the idle quality issues on those carbs in motors with big cams, where the rules didn’t prohibit it, is to install 4 corner idle....... which nearly doubles the idle fuel flow capacity.

You can’t do that for Stock....... but you could get creative with some replaceable idle feed orifices installed in the secondary side of the main body or baseplate.

While a std performance Holley carb with a normal 2 corner idle circuit would definitely be an improvement over the factory 383 carb in terms of low speed operation........ if you swapped it out for a carb with 4 corner idle you’d see how much better still it could be.

Right now, with the tiny ifr and lack of any additional idle fuel flow from the secondary side....... I’m going to say the low speed operation bar is set extremely low.

First thing I’d do is get a performance calibrated 4160 carb body(like a 3310) and see exactly how the secondary idle ciduit is done on one of those, then duplicate it....... along with using a tunable secondary metering plate.

In the past, when I’ve used those with big cams, I’d just remove the plate and swap to a block...... and then add 4 corner idle.
Which is also what the guy did to the carb in the link.
However, neither of those things are legal for Stock.

Can you stall it by closing the mixture screws?

Judging by the pics of the baseplate, the primary slots look rather long.
My guess is....... there is too much showing after you opened the blades more to change the idle speed from 1100 to 1400.

Have you qualified any of the other low speed concerns?
Still starts good?
Idles any better/worse in gear?

Also, what is the idle speed in gear, and what is the vacuum reading idling in gear?

1400rpm is too high to try and adjust the mixture screws. i'm skeptical about being too rich if the only idle feed is two .033" jets. you don't think your pushing thru the needle and seats do you? I have you measured the seat orifice to see if it's in that .075" range? you may have more than one issue going on.

That’s amazingly high vacuum in gear for that combo.

My 383, with slightly more cam was about 7-8” in neutral and 4-5” in gear.

That’s amazingly high vacuum in gear for that combo.

My 383, with slightly more cam was about 7-8” in neutral and 4-5” in gear.

I’d probably start by just lowering the idle speed a bit and seeing how it behaved.

The idle quality now is much rougher. It's not going to like lowering it.

I guess the question that may not have been asked is......

If it was idling fine before, with the blades in the proper position, mixture screws in the proper range, and it was starting well, had plenty of vacuum, etc....... exactly what was the issue you were trying to cure?