I would go with a holley 650 or 750. They are good carbs. A friend had about the same setup and used the 650 and then got a proform 750 center system for it. He had also tried the 850 from a friend and it was way to big.
Keep this in mind
You can always get a carb that is to small to run right. But you can't get a to big of a carb to work right.