There’s a vacuum fitting on the rear of the intake manifold behind the carburetor, or use the driver’s side vacuum port on the Edelbrock carb. Either one will give you manifold vacuum. A properly running motor with no leaks should see about 18-20" of vacuum at idle, lower than that you probably have a leak. The vacuum gauge I linked comes with good instructions that explain all this in more detail.
Also, My headlight doors technically work but leak vacuum profusely (and make the engine run poorly) if I have the system hooked up. I’ve disconnected mine until I can rebuild/replace with electric motor conversion.