Quoted:
My intent is to make it legal. So a type 7 FFL dealer can stamp and transfer to me. I take it.
No. If you build the rifle with the intent to keep it and play with it, you need not have an FFL nor do you need to put a serial number on it. BATFE SUGGESTS that you do so, but it is not a legal requirement.
If you later get tired of your toy and decide to sell it, you still DO NOT NEED to put a serial number on it.
You NEVER need to serialize a firearm made for your personal use.
edit:spelling