First, I thing is I am not a fan of "DUMP CARBURETORS" also called secondary carbs. The reason is the fuel stays in them until you put your foot to the floor on the highway and hold it there longer than you can legally if you want to replace all the fuel in the bowls with fresh fuel. How many people drive their cars that hard on the street?. So what happens to the crappy gas you have when it gets old? it can do carburetor and engine damage on top of poor running. FYI the front factory 2x4 Buick AFB has no mixture screws but it does have a fixed idle circuit so the gas is being used when ever the engine is running. Before you play with the carburetors get that ignition working right, read that section first.
The more carbs you run the more important that they have nice tight shafts and perfectly fitted butterflies, if not, your engine will not idle down. Most need to be restored, they are old and will need more than a "KIT" to bring them up to like new. Many of the old aftermarket intakes have sloppy drill mounting holes so those can cause linkage binding, make sure everything is straight and aligned.
On a 4-2 or 6-2 log manifold with Strombergs, start with stock jetting, adjust the idle stop screws so they just keep the butterflies from binding in the closed position , idle mixture screws one turn out to start, bolt them on the intake. For set up purposely put a spring on each carb so butterflies are completely closed. With air horns off and 2.5 lbs of fuel pressure (electric pump) adjust the floats until you 1/2" of FUEL LEVEL measuring from the top (engine turned off). Now install the air horns and start the engine. With a small vacuum hose 1 or 2 foot long, stick one in one of your ears and hold the other over each carb, listen to the sucking sound and try and adjust idle stop screw as needed so all carbs have that same sound and hopefully engine is not idling too fast. If it is you will have to back off the idle screws to get the idle down. DO NOT put your head, any body part or clothing over the carbs, a backfire could set you on fire! .. Now adjust all your linkage to fit the carb levers and remove springs you added. I prefer straight linkage(on 4-2 and 6-2 setups), progressive linkage is not going to change the gas mileage and will take both feet to open the outside carbs! We do like progressive on 2-4 (dual quad) and 3-2 setups. Most 3 two barrel setups are not enough CFM for all but the smallest Nailheads. Three Holley 94's or Stromberg 97's are less than one small 4 barrel.
Ok now you have it idling and running good but when you open the throttle you get lots of black smoke.. You have too much gas squirting into the intake with all those accelerator pumps. You may only need the two middle carbs (with 6-2) so disconnect the pump linkage of 4 outer carbs and see if you works without hesitating when you crack the throttle. If it works remove the leather off the accelerator pumps on the 4 end carbs and put the linkage back on so your power valves that are in the bottom of the chamber still work. If it hesitated with only the 2 center carb accelerator pumps hooked up then try running the 4 outside pumps with the center ones disconnected.
Max Balchowsky loved running 6-2's on his race and road cars, he told me he got 18 mpg driving to the track when everyone trailer-ed their race cars! I noticed a big change in fuel mileage in the mid 1980's so doubt you will get anything close to that.
Low fuel pressure needed with Stromberg and Holley 94's requires a large fuel line and your regulator should be close to the carbs. If you try and run higher pressure the carbs will flood. Other carbs will work fine with higher pressure but I don't like running more than 6 lbs. If you are using a Holley 4 barrel they can take more pressure than 6 lbs BUT with a Holley, you also get frequent rebuilding, blown power valves, gas leaks and engine fires so we leave those for the Chevy guys to use, it keeps their population down.. :-).