FWIW, when I was fooling with Weber carbs decades ago, I found that joining all the intake spuds together with nice hard AN lines produced zero total vacuum. Using one line to any intake and adding a 15" long RUBBER hose produced consistent enough vacuum to activate distributor vac. advance. Using a single hard line gave zero readings again- apparently the rubber damps the extreme vac/pressure pulses in the intakes.
Tuning is vital- most Weber system either produce 12 mpg and good power, or 16-20 mpg and no power. Getting the best of both requires much fiddling. Fuel pressure seems irrelevant to good running. Do NOT use ANY kind of foam air cleaners: Webers meter fuel into airflow going in both directions so the strong pulses produce a cloud of stand-off fuel-air above each carb throat. This soaks into foam air cleaners, then a cold-start backfire can catch the soaked foam on fire. ONE carb should have a Weber-built startup attachment in place of a choke.
Also, many Weber carbs have been sitting in storage for years or decades, which dries out gaskets, o-rings and seals even on carbs that have never has gasoline in them. So NOS carbs may leak profusely on start-up and need complete rebuilding. It's things like this that give Webers a bad name & try one's patience.