You connect a vacuum signal to the petcock so that it's open while the engine is running. You'll probably have a vacuum line that's T'd from cylinders 1-4 and cylinders 2-3 to provide a semi-steady vacuum signal.
The 3/8" ports on the top rail of the carbs isn't a vacuum port, it's a bowl vent for the stack. You can either connect a vent tube(s), or leave them open. If you leave them open, might be smart to put a screen over them or something, so bugs don't nest in there.
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