carburator help

I think 850 cfm is too big in my opinion. I would think a 750 would give you much better off idle throttle response. If you decide to go with an 850 and are set on that size, I would use a vacuum secondary carb.
Don't get me wrong, I'm still fighting a 650 Holley HP on my 318 build but I'm pretty sure if it was smaller, it would have worked better.