Check with 4thIDVet, he's went through the process. Form 6 is simply the application (and license once approved) to import gun parts, ATF will send you the forms.
If you are serious about this, you can sell every one you could import. Supply of original mags has dried up and many people want these. If you were only wanting to grab a few for yourself you might want to consider a bulk buy and make a few bucks.