OK so do you still have the same symptom? Can you keep the engine running by pumping the primer pin? The symptom about revving up a bit before stalling is most likely a fuel starvation issue, but there is a chance it is running on one cylinder then the second cylinder kicks for a bit…I would have to hear the engine misbehaving before deciding if you have a fuel or ignition problem…Again, “most likely”, it is a fuel starvation issue.
So, you have resolved the messy intake sealer issues and replaced the gaskets? The rubber pressure valves are in good shape and not stuck to the reed plate preventing them from opening under pressure? It seems like you are saying that the same symptoms still exist, and the tank isn’t pressurizing for some reason. I think it is safe to say you have ruled out the tank after trying two or three other tanks. Did you check the rubber hose from the engine’s fuel connector to the manifold? Perhaps the air nipple in the engine connector is messed up/clogged up. Perhaps the air pressure nipple in the manifold is plugged up? Someone before you may have attempted to convert this engine to a single line tank, they may have purposely plugged off the pressure nipple in the manifold.
Back to the intake gaskets…There are two different gaskets used on the manifold, even though the bolt pattern is the same and they could easily be mixed up. If the wrong gasket is used behind the reed plate, this will create an air leak between the two crankcases…This air leak would create a lean condition, and also result in weak pressure pulses to the fuel tank air line.

With the engine’s air line pulled off the connector, you should feel constant pressure pulses, no vacuum pulses though….