Carb CFM.. Increase?

I’d start off with the 650, see what vacuum level it pulls at full throttle run. And make an educated guess to how much larger you need if any.

The cid x rpm / 3456 = cfm
Is a good appropriate estimate of how air will be consumed by your engine but the problem is that number has no direct relationship with how carbs are measured. An 850 carb doesn’t move more air than a 650, say you engine consumes 750 cfms of air and fuel. Both 650 and 850 will pass 750 through just at different vacuum levels, “restriction” 650 will probably be above 1.5 hg and 850 should be below 1.5 hg. Giving more or less pumping hp loss.