vaccuum leak?

If those holes went thru, your leak would be so bad it wouldn't run at all. I had the same rough idle symptoms in my 69 Dart 225, which drove me nuts for decades. I tried several rebuilt carbs, intake manifold, head rebuilds, even a whole new rebuilt engine, plus electronic ignition. Several mechanics said "valves" (but see above). I swore off mechanics after maybe six couldn't fix it over the years, but did take my money. I finally tried another Holley 1920 carb. It ran like a dream. Almost instant throttle response at idle and pulled hard. Conclusion - many poorly rebuilt carbs being sold. TBI is the way to go, but currently that is "roll your own" for Slants, as far as I know.