There is a vacuum fitting/port/nipple/thingy under the front of the carb body, near the base and adjacent to the idle-air mixture screws. I believe that is the full vacuum port on one of these carbs. The other one, sticking out of the passenger side, is (I believe) the ported vacuum often used for the distributor.
Was going to say the vacuum tree as well, as that will always have full vacuum. Not sure if the readings vary slightly between the tree (plumbed to a single intake runner) or the carburetor (plumbed to a common plenum area) but they should both be close and give at least semi-consistent up and down readings on the gauge.
Your '70 won't have a true "tree" (at least not from the factory) but would very often have had a block fitting with multiple vacuum hose ports sticking out of it, screwed into the intake manifold in the same area behind the carb that a later model ('73 and later) would have had a fitting that looked more like a tree.
Remember too, that the most vacuum is not always the best. Yes, it's most likely the best, but when it comes down to every individual engine, you will want to do the tune-up "dance" where you alternate between adjusting the ignition timing, idle-air mixture and idle speed screws.
You're looking for high vacuum, but also the most steady vacuum and best idle quality.
Which means that the two idle-air screws do not necessarily need to be the exact same number of turns out. This will vary by carb and engine.
Sorry if you knew all that already. Just wanted to make sure in case this was your first time.
Paul