If at all possible, try to get written ATF approval.
If push ever comes to shove you really, really REALLY want to be able to demonstrate that ATF approved it. So here's what I suggest: Make some extra copies. Lots of them. Laminate them. Then mail copies to your five closest friends, preferably scattered to the four corners of the globe, three more to your lawyer, two more copies in safe deposit boxes, bury three more, scan one copy to your hard drive with online backup, another copy on a flash drive, and mail three copies (from different post offices and on different dates, registered mail, return receipt requested) to yourself with your signature over the flap on the envelope (make a small note on the outside of the envelope and do not open them unless in a court of law for an Ah-HA moment). . . oh, yeah and find a couple more ways to safeguard a few more copies.
Did I mention making a couple of extra copies?
I'm a lawyer, but I'm not your lawyer. If you need some honest-to-goodness legal advice, go buy some.