As I'm sure you're aware, 850 cfm is the flow rate at a set pressure differential across the carb and normal air inlet density(The max it would flow on the inlet of any roots style blower like your examples). When you force air through a carb in a blow through system, it flows more air molecules since the pressure differential should be be higher and the density of the air is higher, especially if using an intercooler before the carb inlet. When modifying a carb for blow through, one of the significant mods is to increase the size of the needle and seat assembly to flow more fuel to keep up with the increased air flowing through the carb. BTW, I have run a blow through system, so I have some first-hand experience. I use EFI on my turbo engine now and would not use a carb again on a boosted engine. They work well, but EFI is better for a street/strip car IMO and I drive mine more on the street. That being said, the size of the carb that would be needed by the OP cannot be determined because a HP goal was not stated. A call to one of the previously mentioned carb vendors that specialize in this type of carb would be the best course of action.