[LS short box]

If all else fails there is a guy over on Corvetteforums.com that goes buy "Lars". His is highly regarded as a rebuilder of Quadrajets.
Many many moons ago as chev line tech we rebuilt many quads. But we would take them apart in the morning or afternoon and then let them soak over night in the carb cleaner bucket. I kind of wonder if any shop uses that stuff anymore? It was nasty. Didn't want to get it on your skin as it might create a chemical burn.
The next morning rinse them in tap water and reassemble with the rebuild kit.
One more thing. Check to see if there is a bunch of play in the throttle shaft. It can create a vacuum leak if badly worn.