what CFM would you use here

CFM ratings can be apples/oranges. Different levels of depression were used by different OEMs and carb manufacturers. Here's a little light reading http://www.candsspecialties.com/ratings1.html

With a mild 360 with street gears something like an Edelbrock 1406 will probably fill your needs. If you're inclined towards Holleys, try this http://holley.com/applications/CarburetorSelector/CarbSelection.asp