Weber carburetors (Italy) are great, if you are looking for a wide range of engine RPM, and have unlimited amounts of money for purchasing tuning parts. Or if a new set-up has been dialed in for ones specific application, buy that rather than trying to re-invent the wheel. If one starts with a "clean sheet of paper", one had best have available a bucket full of jets, air bleeds, venturi, aspirin, etc. for tuning purposes, but the finished product runs well.
Have little experience with the Holley-Weber (USA) progressive two-barrel, but since these came originally on smog-emission engines, believe tuning on a non-original application might be difficult. Do not know availability of tuning parts. Really newer than our area of expertise.
If one is looking at newer carburetors, Carter AFB's are hard to beat, as Carter made them in sizes from 400 to 950 CFM; and offered much in the way of tuning parts (jets, rods, springs), most of which is still fairly available. And obviously, they are progressive. Have run the 400 successfully on many 4 and 6 cylinder engines. Larger sixes (and higher RPM) can use 500 CFM or???
For vintage speed, multiple single carburetors look and can be made to run very well. Generally best to either start with carbs designed to run in multiples (ie Twin-H Hudson) or carbs with mechanical power circuit (ie Carter W-1). Vacuum power circuit carbs can be used, but tuning is less straight-forward.
Jon.
