If it is a 440 manifold there's your problem. It has to be sucking air and oil into the cylinders from the valley. Forget your plugs see if they're only. Take your carb off look inside the manifold for oil. If there's oil in the manifold get rid of it and get a 383 that is a low deck put it on and put the car back on. The carb is not too big it is a spread bore with tiny primaries. Vacuum secondary is only going to allow the engine to pull as much fuel and air as the engine needs in no more. Change the manifold for the right one put it back together and you will probably be fine.