So you want me to buy every carb option and throw away the ones that do not work well? Really? 770 cfm is 30% more air than this engine can possibly consume.
Do NOT get caught up in the CFM numbers. The carb will flow the air the engine needs. With the adjustable air valve on the secondaries you can tune that up.
Again, like many things carb related, the math to calculate CFM usage is at best, silly. It's really pretty much worthless. It doesn't take into account what I said above, and it also doesn't account for pumping losses and things like that.
Drop the 650 on there and work on your tune up. Ignore the math.
In fact, my carb is considerably larger CFM rating wise than the math calls for. Think about tunnel rams with dual carbs.