I would tend to pull the Holley and replace with a new Edelbrock 1406 electric choke carb. Holley's tend to run rich and have tendencies to leak. You can change jet sizes but still have a mixture problem---float level, air passages, mixture needle and seat, etc.
I have a similar problem , only at idle but due to too much cam. Can get the idle vaccum high enough. Plugs don't foul but at idle the exhaust sets off my smlke alarm in the garage before I can get the Blazer out!!!
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