Vacuum

no sir, my days of owning slanty cars is long over.
But, if you remove your air-cleaner housing, you might just find an appropriate nipple sticking out somewhere, or at least a place to put one.

The nipple you mentioned is called IIRC, a Vacuum Tree. I have seen them with one, two, three and four branches. Usually if it has three or more, it will have one large one that fits the booster charge-line. and sometimes the doubles have one as well.
If you cannot find a ready-made access port on the plenum, then you might as well try using the port that you do have. Maybe it will work for you. I seem to recall that, a long time ago, one of my slantycars did have the booster plumbed there. But man, that is a long time ago.
If your engine idles well in gear, especially on choke, with a quick warm-up, and musters adequate vacuum on the one-leg, it will probably be fine.