Hi folks, can now update with findings for my vehicle as well.
After a long saga, I pulled both Webers and the intake manifold and did a full detail of the carbs and ensured various passages were clean, along with some basic tinkering (following Gordon's advice carefully to avoid a mishap). A detailed cleaning of both barrels and the edges of both butterflies of accumulated gunk improved the closure of the butterflies. Next, the intake manifold got a once over including a new brake booster vacuum hose as the old one was dry and cracked slightly at the intake connection. My conclusion is that this is the root cause of the slightly lower vacuum reading from that offending carb barrel. Everything put back on with mostly new hardware and full isolator mounts instead of those billet mounts with simple o-rings.
Unfortunately this did not fully solve the issue! Moving to the ignition system, which I should have done before everything else, I found that cyl. 4 was not firing at all and traced it back to a faulty Marelli distributor. Having had put so much effort in at this point, I decided a 123ignition programmable dizzy was the obvious next gift to myself. That got cyl. 4 firing and we are now in the tuning/tweaking stage.