Ive done the 400 and its predecessor many times with a set of £70 gauges-The first thing to do is re-build the cabs with a jet-set and O rings. Oh and make sure they are clean!
As you have eluded to-the gauges measure the air-flow/suction-so have nothing to do with mixture setting-thats another black art if you have the correct jets fitted and seals.
From my experience, the most common-fault with those carbs is blockages and leaking main-jet seal-the later causes too much fuel to be sucked up the emus-ion tube - leaving you with a poor metered fuel mixture.
Get all gauges reading the 'same' and you are 80% there. The general 'noise' is an indication of where all 4 'motors' are sharing the load-as said, do the fast-idle and 3k-if one works, the other should. It does take a bit of trail and error- and make sure the tubes (usually brass) are seated and air-tight on the take-off points. Remember, as the engine heats, the readings will change-so just aim for equilibrium-then take it out for a run and re-check.
Just as a side-note- Rich running? How do you know this- plugs back? My first point of call on rich is to remove the air filter- long story short, my sea stand-off bungs where too low and blocking the air-intake. it was fine on the guages and tickover in the garage- then i stuck my fat-arse on the bike and it would pull ....