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[ChevyTech]

Being there is only a miss at idle, it sure sounds like a vacuum leak.

I like to use a needle nose pliers to pinch off vacuum hoses, one and at a time, and listen for a difference.
I also use a 3 foot long piece of heater hose to listen for a hiss as I move it around the engine.

Vacuum can leak through components such as the brake booster or charcoal canister purge valve.
Don’t over look the possibility of the EGR valve being stuck open a little.

Look at the edge of the intake manifold and see if the gasket looks uniform and in place.

The intake question is interesting because I recall my dad telling me that when he bought the intake gaskets there were none for the front and rear so he filled with RTV and put the intake on.

That’s the way they are done on 1980s V8s. (No front and rear)