Buck - the square-bore with either the same size or slightly different sizes applies to all carburetor makes.
JW - there are a couple of aftermarket 400 CFM AFB carbs (9400s, 9410s) that as you suggested, are in GREAT demand (read EXPENSIVE) for 2x4 set-ups.
Additionally, there were a very few 380 CFM and 400 CFM AFB's produced for factory installation (one was the Sprint option on the 225 CID MoPar, another the 271 CID Barracuda). Most of these are so rare, I have never even seen one! As far as price is concerned on these, the last one I had brought $2500 several years ago. After the customer paid, he told me he had been looking for more than 3 years, and mine was the first one he had ever seen. He sent his private plane to the local airport and I turned over the carb to his personal pilot. Most folks do not wish to play in this ballpark, which is why I don't suggest these in my posts.
Take a look at (D) and (F) in my post above.
Jon
