If you are running the Original Breaker Point ignition with a factory four speed with power brakes, you will have many vacuum hoses and vacuum ports. Vacuum is supplied through several openings in the manifold and carb base front, rear and sides.
The "Tree" nytrainer mentioned is actually called a "Ported Vacuum Switch." It has at least five ports that connect to hoses. The Vacuum ports open and close based on operating temperature.
The Four speed has a vacuum control valve at the back of the intake manifold and controls the advance and retard ports on the breaker point distributor. Lots of vacuum ports and hoses. Make sure the ported vacuum switch has complete integrity and the same for the control valve at the back of the engine on the passenger side.
If all the hoses and ports are sound, then you need to check the gaskets for the carb and the intake manifold.
nytrainer is correct that there is a hollow bolt at the right rear of the carb that is a source of vacuum for the vacuum control valve.
A bad valley pan gasket will possibly cause an oil leak, but not cause and engine vacuum leak.
After all this I assume that you have a stock cam?
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