CFM= CID x max RPM divided by 4567.

So if your running a 406, with a valve float rating @ 7200 rpm, then you need 640.07 .
A 650 is what I'd start with, unless you have a massive lift cam, and then jet up if need be.

Start with a fresh set of plugs- run them under YOUR normal running conditions- pull one from each bank and get a reading, they should be a light brown to tan. White is too lean- jet up, Black is too rich - jet down.