If I was building the engine you have and I already had the 850 CFM carb I would definitely use it, and tune it to work. If I was looking to buy a new carb for that build I would go with a 750 CFM carb. Another question is, are you talking about an 850 double pumper or an 850 vacuum carb? An 850 DP is very difficult to tune with a smaller engine, unless you have a manual transmission, or a high stall converter (3,500 RPM+), and low gears, like 3.91 or 4.10. A vacuum carb is much easier to get working. If you build that engine with 3.23 gears and a 2,500 stall and put a double pumper 850 on it, you may never be able to get it working optimally.