That book is great, The picture opretty clearly shows the difference between the venturi size, it's a little hard to tell when you dont have the book at hand though. And for the record, some electronic carburetors from the 80's had a stop on the butterflies, that would limit the max air-flow, it is mentioned in Doug Roe's book too. They could easily be measured at around 650 cfm.
Btw, it dont matter if you have a 750 or a 800 on a 403. It will never eat that much air anyway unless you really spend some money on it, or rev it to 10k rpm ;-)