305 QJet Problem

[76C10Stepside]

Finally got back around to working on my truck. It's a 1976 C10 Stepside with 305-V8 with 2Jet carb, transplanted from a 76 Caprice, and T-350 auto that I think is original to the truck. It ran like a champ for first 7 years. Rebuilt the top end second year and it runs very smooth.

Here's the problem: The engine wants to stall when I give it gas at a stop or in first gear. Once I get it going (like second gear), it runs fine. It's acting like there's a vacuum leak, but I can't find one anywhere, and no telltale sucking sounds. Pretty sure I already tried spraying starter fluid around to check for air leaks last time I worked on it.

Got a rebuild kit awhile back and replaced the throttle pump skirt, inlet valve and bowl gasket. Inside of carb is very clean and all parts look fresh. It squirts a nice twin jet of gas, so I've ruled out the throttle as the cause.
Today I cleaned the carb and carb base and replaced the carb-to-base and base to intake gaskets. The rebuild kit said to adjust the mixture screws on the separate base 2 turns out. It still stalls when I hit the throttle (did not try driving it yet). Tried adjusting out another half turn, no difference.
Crawled under the truck and checked the vacuum tube that runs from intake manifold (not on carb) to trans, and it seems fine. There is a short piece of hose that connects the steel line to the trans. I did not pressure test it yet tho.

All vacuum ports on the carb are either properly plugged or have good hoses on them. I don't think there are any vacuum leaks on the carb or intake manifold.

Can someone tell me what effect adjusting the mixture screws out too far will have on the throttle response, vs. adjusting them in too far? I would not expect mixture to have too much affect on throttle response, cause they are for idling. Any other ideas what the problem might be?

Thanks, and hope you all are enjoying this nice hot summer!