That is true, but my professional take on the situation is to err towards the side of caution and avoid a constructive possession accusation by simply not owning the parts.
Even if you are legal, it will not stop you from getting hassled by those not in the know. The best example I can illustrate is at lunch today when I was talking with a few members of the local swat team. One of them had went to New York City and was checking his bag to fly home, declaring a firearm. Gate agent said where's the NYC pistol permit, he said he didn't have one.
....thats when the airport police showed up ready to arrest him, had he not been carrying his department ID. He was to say the least - flabbergasted at the sheer insanity of it.
If you do build it as a pistol, you will have to get documentation from the dealer that the 4473 was transferred on "Other", proof that it was new and has never been stocked, etc.
You might be right, but you might go through the wringer to prove it. That's all I'm saying.