what CFM would you use here

so this engine is bone stock, with a slightly hotter than stock cam? a 650 would probably do the job. I had several 318's with fairly moderate cams that really liked 750's...but again being that close to stock a 650 will probably be the better route for a daily driver.

as far as an adapter goes...you'll see alot more from a nice edelbrock performer or equiv. intake, and carb, than you will with just an adapter and carb. I would deff stay away from the adapter plate. I see some bogging and choking issues with that. You'd be trying to cram all this new CFM you spent money on in your new carb...down the same 2 bbl hole in the intake. not a good combo.