I belive the casting are for the 800 CFM , But on the carb's that are on turbos the secondarys are limited on their opening so it flow 570 CFM , this is where doug Roe book shows you can trim the "tang" to have it open more to give more flow.
But go to big and the car will buck & miss (I have tryed this) . What I did was drilled and taped a hole and inserted a screw than you can adjust the total CFM , If you go to big you can adjust it so the blades do not open has much , to small then you can adjust it so they open more. (50° angle on stock , hot rod tech artcile suggested 70° ) I found 70° to much . But every car is a little differant.
I had the same problem with the big holley , these cars seem to like a total carb size around 600-650 CFM.
Thats my two cents.