From personal experience, I use a gasket below the phenolic spacer and above. When the carb was installed without the one on the manifold, I had a vacuum leak.
To check for vacuum leaks, take a propane torch and turn on it on without lighting it.
With the engine idling, move the torch around any possible vacuum leak areas like the carb mounts, bowl and base connection, wiper connection, intake manifold connections, etc. If the engine rpm changes at any time, you'll know that you've found your leak.
