I assume you're using the original Rochester 2-BBL carburetor? Pull the carb, and see if you've got an exhaust leak where the heat riser passage goes around the carburetor base. Those older Rochesters used to be notorious for burning away part of the cast iron carb base, and sucking up raw exhaust gas from the heat riser passage, which actually ran exhaust gas against the base of the carb under a thin stainless steel isolation plate. The symptoms were exactly what you're describing- - -rough idle, and a very hot intake manifold heat riser passage from the constant flow of exhaust gas pulled in by manifold vacuum.
Jerry

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