[Oldsguy]

Mike, it sounds to me like you are on the right track. I wonder about the inconsistent vacuum. With everything plugged, except the advance cannister on the distributor I assume, you shouldn't have a leak. With a TH400 you would have a vacuum operated kickdown, that might be it, or the accumulator for the brake booster assuming you have power brakes. Like you said, it might be gaskets, or could be the throttle plate bushings on the carb. If the bushings are worn, you would have a vacuum leak that would come and go. Would also effect the carburetor performance too. One other thought might be a worn timing chain, have you double checked you timing? It might be jumping teeth.