I am sorry but you are incorrect, Boomfab.
Title 27 479.102 states:
"Sec. 479.102 How must firearms be identified?
(a) You, as a manufacturer, importer, or maker of a firearm, must
legibly identify the firearm as follows:
(1) By engraving, casting, stamping (impressing), or otherwise
conspicuously placing or causing to be engraved, cast, stamped
(impressed) or placed on the frame or receiver thereof an individual
serial number. The serial number must be placed in a manner not
susceptible of being readily obliterated, altered, or removed, and must
not duplicate any serial number placed by you on any other firearm. For
firearms manufactured, imported, or made on and after January 30, 2002,
the engraving, casting, or stamping (impressing) of the serial number
must be to a minimum depth of .003 inch and in a print size no smaller
than 1/16 inch; and...."
You, as the one who chops the barrel, removes the fake flash hider, puts the 11.5" upper on your AR15 are considered the "manufacturer" of the NFA firearm. Therefore you are indeed required to mark the weapon in accordance with the above quoted regulations including the minimum 1/16" letter height. And since you are re-using the original serial number, you are not required to assign a new one to the weapon. However, if you are manufacturing a brand new weapon, such as building a full-auto AK47 from a flat, then you would also be required to imprint the weapon with a unique serial number, caliber, etc.
Title 27 of the CFR online, 479.102.