360 Questions.. pardon my ignorance.

CoolDave said

It’s hooked to what I’m pretty confident is the correct port on the carb, there’s a vacuum tree on the manifold that’s used exclusively for the brake booster and vacuum gauge when I’m using it.

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Not according to what you wrote earlier - timing at idle speed increased when you reconnected the vac line to the distributor. You will have manifold vac at idle, not at WOT, and ported vacuum works the opposite, so it pulls in more timing when vacuum drops because some lead foot just hit the floorboard.

One more thing I just thought of - I have a Brawler carb that has two vac ports out the front and one is ported, the other is not. That about drove me nuts until I figured that out. IIRC the ported connector is the larger of the two, which is even weirder because it's too big for a typical vac hose to the distributor. I was connecting my vac guage to the larger port thinking it was like the one on the other side for the PCV. Imagine my surprise when my engine would run with zero vacuum.