[68gmsee]

When something like this happens and I'm not sure I always do the easy stuff first before any engine disassembly or dismantling of parts.

Timing, fuel delivery and vacuum are some of the first things to check and you've check that but on top of this, check for flooding by visually checking if there's excessive dark gray to black smoke coming out of the exhaust.

I always block off all vacuum ports on the carb and intake manifold: vacuum advance, power brake booster, transmission modulator vacuum source, etc. to see if the idling improves.

If all that checks out, fuel contamination could also cause this. When I suspect this, I use a one gallon container with fresh gas placed under the fuel pump and run a short section of fuel hose to the pump intake from the container.