A lot of good replies already. My is also that every time I have added CFM to these larger engines, I have seen more HP.

Do you already have the 850? If so, or if you have access to an 850, why not strap it on and see how much vacuum it pulls at WOT? That would seem a good starting place to me.

Lacking any real engine specs or it's intended use, my first reaction is MORE CARB. 4500 all the way. But that's just me.