Help with vacuum port identification

There are 2 types of vacuum connections on a carburetor. Ported, or timed vacuum, and manifold vacuum. Ported vacuum is sourced from above the throttle blades. Manifold vacuum is sourced from below the throttle blades. It is not always apparent which is which just looking at the carburetor. Fortunately, it is easy to tell the difference once the carburetor is installed, and the engine is running. At idle, if you uncap a ported source, nothing will happen as ported vacuum is non existent at idle. As you open the throttle, ported vacuum increases quickly. Leaving a ported vacuum nipple open will not create a vacuum leak, just a source of unfiltered air. Uncapping a manifold vacuum nipple at idle will be very obvious as it will hiss loudly and the engine will run rough because you have created a vacuum leak.

The power brakes and transmission vacuum modulator require manifold vacuum connections. Those connections come off the back of the intake manifold, NOT the carburetor. Technically, you could connect to the carburetor, but no engine came from the factory like that. There should be 2 threaded holes in the back of the intake for those connections. This is a picture of a Stage 1 Buick 350 intake, but your intake should be similar.



#2 in your picture is definitely the PCV connection. The other front connections could be ported or manifold vacuum connections. It is important that you determine which is which. The distributor vacuum advance can be run off ported or manifold vacuum. Nailhead engines usually used manifold vacuum. If you use an air cleaner with a hot air door in the snorkel, it requires a manifold vacuum connection to the sensor. The 2 connections at the back of the carburetor are manifold vacuum. The climate control system uses manifold vacuum to operate the temperature doors. Usually, there was only one connection at the back of most carburetors that went to a vacuum storage container on the fire wall. Cap off whatever you don't use.

Not sure what you are referring to when you say you have 3 threaded ports on the "inlet" I only see your fuel line connection.