Just issues of engine stall when choke is wide open at idle

What list # are your carbs? I'll look them up in my Holley master parts catalog and give you he stock jetting for the center carb and the part number for the outboard mixture plates and hole stock hole sizes, hopefully.
Me thinks someone may have mess with them before you got them
On the power valve the vacuum rating shouldn't have any affect on the idle unless you have the idle speed screw adjusted up far enough to make the center carb idle on the transition circuit, you can check that by removing the center carb and flip it over after draining the gas out of it and see how far the throttle blades are opened in relation to the rectangular slot in the front center of the carb throttle plate if you can see any of the slot below the throttle plates it is to far open and idling on the transition circuit
I check the power valves by removing them and then use my mouth and tongue to suck on them and make my tongue seal against the center hole, if the valve stays closed with my tongue on it is good, if it doesn't stay closed it is bad A ruptured power valve will make it idle rich all the time due to leaking fuel into the carb.
I use a vacuum gauge to check the idle vacuum and install a power valve that is one to two inches lower in vacuum, if it idles above 8.0 inches I use the 6.5 rated valves If it idles at 5.0 inches in gear like my old pump gas stroker motor did I would use either a 3.5 or 2.5
I used both in it and it work the same, not so on the 4.5
Do you have a pin vise and a good small (.011 to .060) set of drill bits? If so use the drill bits to see what size the bleed holes are on your carbs, the center ones are the idle bleeds and outboard ones are the high speed bleeds Post them on here and I'll look up which size they where stock , if I can find them in my catalog
I've owned, tuned and driven a bunch of stock 340 and 440 six pack cars with the stock carbs, they would idle great and drive okay with the stock jetting and so on. You can make those motors run a lot better by proper jetting and tuning
Open the outboards idle mixture screws to 1/2 turn and the center to 2 full turns out from bottom and then use the center carb mixture screws to get it to run as lean as you can when warmed up and set the speed idle at or below 1000 RPM so the center throttle plates are not opening up to the transition circuits
It is snowing here, hopefully it will not snow after tomorrow, we need the water but it is mid March
Good luck on this, you can fix it

center carb is 4375 outer carbs, front 4382 and rear 4383. I dont have the listing numbers as hard to read. Far as the base plate is concerned front base plate has been messed with as it was redrilled with new inserts to fit the screws. It was stripped and had to be drilled out. The hole is bigger than rear carbs and needle sticks out much further. I figure this one is easier to adjust since it's in the front. New base plate have been tough to find.

Your picture of the right side show the vacuum port for the rear carb booster is open. All three carbs need to have the vacuum ports connected through a T fitting. It looks like you have the center and front carb connected with a hose and nothing to the rear carb. They all three need to be connected together.

they are all connected correctly. What you see is a vacuum hose that goes into the choke pull off disengaged so the flap stays closed.. I put a screw in it to stop it from air leak.

center carb is 4375 outer carbs, front 4382 and rear 4383. I dont have the listing numbers as hard to read. Far as the base plate is concerned front base plate has been messed with as it was redrilled with new inserts to fit the screws. It was stripped and had to be drilled out. The hole is bigger than rear carbs and needle sticks out much further. I figure this one is easier to adjust since it's in the front. New base plate have been tough to find.

The center carb. came with Holley #63 jets in it, the 4382 outboard carb should have a Holley part # 34r-6153-34 metering plate in it and it should have had a .052 fuel inlet size in the bottom of that metering plate, the other carb # 4383 should have had a r34-8153-35 plate number in it with a .071 fuel inlet size hole on the bottom of it.
I have heard a lot of people on here say that drilling those holes out to.086 works good
My set of original six pack 1970 carbs like #63 jets in the center carb and .096 air bleed jets in the one side and .092 size air bleeds in the other. Those plates had the original holes plug off and where drilled out and tap to use Holley screw in air bleeds. that set up worked real well Once I got the fuel system straighten out with the proper size pre fuel pump fuel filter size it didn't want any richer jetting than it had
I did try richening up the out board carbs jet sizes by .004 and it slowed down, I didn't try leaning it down though
All the stock six pack center carbs I worked on didn't like a lot more jet size in them, maybe one or two sizes up or down, no more than that though
Take a look at your outboard carbs. metering plate numbers, both of them and post it on here to see if they are stock or been change like the center carb jets have been
Make sure that the fuel feed holes(channels) for the power valve haven't been plug off also
Let us know what you find

To all, I just posted on you tube on this issue. One thing I did discover is that plugging the air bleed holes on the outer carb with wire actually evened out the idle. I took it for a quick drive though it does not stall, its definatley running rich and some loss of power.
Far as the jet size, it's a 65 due to the size of the cam and is stroked out to 512. More and more I think it has to do with center carb not working properly. I should not have plug the outer bleed holes.
I am trying to keep the original metering block but I might just change it to 134-203 replacement and see what it does. I am pretty sure the outboard carb metering plates are both 34.

https://youtu.be/8X2MP8jzD3A

Not idle air bleeds, my carbs had .092 and .096 fuel restrictor, IE Holley screw in air bleeds acting as the jets on the bottom of the stock metering plates with the original metering holes plug off

I thought of something else you can check and reset if needed is the fuel, float level. Most Holley like and need the fuel to barely dribble out of the side hole when running at idle speed, if you look closely at your carbs you will notice that the outboard screws for checking the float level are higher than the center bowl is Don
t set them in the middle of that hole, that is to high
Let us know how your doing on fixing this issue, six packs rock when you get them right Which is not always easy

Cab,you got it backwards,the center is higher than the ends,better check your !