When you use a vacuum gauge, you want to measure manifold vacuum. Connect it to the manifold, where the windshield wipers connect.
As for tuning your carbs,
Are they both rebuilt
Are they matched tags
Are the floats set the same
Horay, if so, you are half way there.
Set both mixture screws to the factory setting, which I think is one and a half turns out, check the shop manual.
Set both throttles to about three quarters of a turn past closed. You can see and do this easiest on the bench. Or, set them a little more open. You want the truck to run when started, and the throttles to be close.
Fire it up, and let it warm up fully. Buy a uni-syn. Read the directions.
You can also sync them by listening with a piece of vacuum hose. You want them to make the same sucking noise. After the throttles are in sync, you can slowly and evenly reduce your idle speed. After you are idling where you want, check that the carbs are still "synced".
Now you can turn the mixture screws, again, you must turn them evenly, to get the vacuum gauge to read the highest. The factory setting should be really close.
Now you can hook up the throttle linkage. And you will likely need to set your timing ahead. California Bill's book explains all this to a "tee".
