Is the engine in running condition yet? The larger of the two holes in the photo is probably the one you need to use, but it's difficult to tell because the entire manifold is not in the picture. The smaller port lower on the casting is probably a passageway for coolant used to provide (not enough) heat to the manifold. It won't be able to provide any vacuum.

Now- - - -with two carbs, it's very unlikely the manifold will have enough vacuum to support a PCV valve and power brakes all of the time. I'd strongly suggest installing a vacuum reservoir tank with a one-way valve between the tank and the manifold, and attaching the booster vacuum line to the reservoir. That will give you one or two brake applications even when the manifold vacuum is so low that it can't operate the booster effectively.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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