Small one up high = ported vacuum.That is for the distributor if you don't want vacuum at idle. Small one under the throttle = permanent vacuum. You can use that. The big pipe is permanent too, and used for either closed engine breathing or a servo.
It's better to have a feed for the modulator directly from the intake, there is usually a pipe plug you can remove, but at a pinch you can use the little one under the carb.
If you have modified a smog motor for more performance you may find it idles better with full vacuum, my motor idled rough with ported vacuum so I speed up idle which exposed transition slot in the Holley carb, then mixture screws don't work! I connected to full vacuum , reduced the idle speed screw, jobs a goodun!
After the oil crisis in the early seventies the cars got heavier and made less power, 5 mph bumpers, lower octane unleaded fuel and emission reduction all played a part, fortunately in this country we can tinker with it, as far as I know you cannot in California, dont know about the rest of America
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