You’re probably getting ported vacuum rather than manifold vacuum if you’re using the vac advance. Hard to make much of your results until you get a better signal. Gauges vary from one to the next, stability is more important than absolute number. But 12 is pretty low, my engine idles at more like 20. But again, you need to get a source of actual manifold vacuum before worrying about the number.
Are we talking about a 235 here?
There are a few different versions of the intake, but the hole there is just some sort of pipe thread (NPT). Should be able to remove the fitting and just go the hardware store and get either a brass tee or a barb with the right threads. Do that and report back.
Does it run OK? Will it idle on its own?
Lots of things we could explore to make it run better (vac leaks, ignition timing, carb issues, etc). Getting a vac gauge hooked up to a proper source is a good place to start that diagnosis. Sounds like you’re on the right track.
