I had similar issue when driving below 55 all fine as soon as secondaries opened it stalled out and not start for 10 minutes.

Problem was fuel sock needed to be replaced. having said that, carb, fuel issues are a process. 

What size carb is that?  Might not be big enough for a big block.

Might also be a vacuum leak.  I don't thing the tranny should pull it down 800-900 rpm.

First, get the idle speed down to where it is supposed to be. Check the emissions label(if it is still on the car, on the firewall area above the master cylinder). Should be somewhere around 600-800 rpm IN GEAR. That high of an idle will shock the torque converter when you put it in gear, and cause it to stall, unless you have a higher than stock stall speed converter. Back off(lefty loosey/righty tighty) on the idle screw, with the trans in gear...you'll need a helper. If it will not stay running with the lower rpm idle speed, then you still have other issue with the carb. I agree with Gary about the not being the correct size carb for a big block. What intake is that...it does not appear to be GM issue....

Once you get the idle speed correct, then re-adjust everything you have already adjusted...everything effects everything. It may take 3-4 times of doing the routine to get the best idle. Once that is done, check the adjustment of the accelerator pump(the lever with the pink looking spring just below the fuel inlet). There should be just the SLIGHTEST amount of free-play in it. Too loose, you get a bog when you accelerate. Too tight, and you do not get a full range of pumping action.

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Thank you. I’ll check the fuel sock. Never even dawned on me to check that!

The Carb is a 600 CFM STREET WARRIOR CARBURETOR. Whether it is big enough is beyond my car knowledge. Whomever put it on did so long before me. But I do suspect a vacuum leak may be playing into this as I have already solved other vacuum issues around the headlights and windshield wipers. Thank you!

I don’t think this carb was meant for a Chevy either… but then again the engine is not the original either. The car came with a 427, but there is a 454 in it now.

I thought a neutral idle should be 750-900 but if you’re saying that is more of an IN-GEAR idle then maybe I have been shooting too low. You’ve definitely given me something to go research; I need to know exactly what engine is now in this car and track down recommended idle speeds. Now that I thin about what I just wrote, I should have started there! 🤦🏻‍♂️

Also a good call on the fuel pump level. Ill check it out. Then re-adjust everything again… and test… and make slighter adjustments, and re-test… lol. It is indeed a process. 

Thank you.

I checked on the Holley website and they said that the first thing you should do would be to check the accelerator pump adjustment. Once you've done that it suggested changing the pump fuel nozzles and increasing them one size at a time. It went on to detail other things but you can find those by going to the Holley website and just checking on carburetor stumble.

A 600 CFM carb would probably be slightly undersized for that car but that would not affect low speed operation.

1973 L-82 4 spd

Also looks like you may be using ported instead of manifold vacuum advance.  I'd look at changing over to manifold vaccum which should be a nipple lower on the carb and then try advancing the timing some.

Sounds like you have a vacuum leak.  Not sure unless I'm under the hood.

Interesting. It was running fine before I reset the mixture and adjusted the throttle screw. Not sure how that affects the fuel pump but there is so much I don’t know. I’ll look into how to adjust the fuel pump.

Ah… I didn’t understand. Yes, I had previously but now wondering how much as actual “play” there should be. Going to try both ends of the spectrum and see where that leads me. Thank you.