I need your help with my vacuum tap (and my manifold studs, too).

They are 3/8 pipe threads. Pipe threads have a taper to them. You can chase the manifold threads with a 3/8 NPT tap to clean them out, but do it CAREFULLY- you don't want to cut any new threads, just clean the carbon/rust out of the existing threads. Running the tap in too deep will enlarge the hole, and cause the fitting to run in too deep- you do NOT want it to bottom out on the shoulder, as that could actually cause it to not seal. The taper is what allows it to seal; threads on the fitting still showing when tight is normal. A few wraps of teflon tape won't hurt anything on a vacuum fitting or coolant nipple. (You don't want to use tape on oil pressure or coolant temperature senders since the bodies require a good electrical ground path to the block, through the threads).
Sorry, I've got no help to offer on the manifold studs.