...But I don't assemble it in that order. ...
As an aside, there are two slightly different issues at play.
1) The final configuration must not meet the definition of a Title II firearm. In this case, you're right in that if you don't assemble the pistol barrel while the buttstock is attached, you aren't constructing a shoulder-fired rifled firearm less than 26" overall and/or with a barrel under 16".
2) You cannon construct a concealable, sub 26" barreled firearm -- or any firearm with less than 16" barrel(s) --
FROM a Title I rifle, and that's where you're running afoul.
It is a lousy quirk of a really badly constructed law (NFA '34). The SBR & SBS portions of the law make almost no sense at all as the final version was (blessedly) passed after having been stripped of its similar restrictions on handguns of all kinds.
The "made from a rifle/shotgun" portion was included as handguns (and any "concealable" firearm) were going to be rendered illegal by dint of exorbitant registration fees ($200 in 1934 would be something like $3,300 today) and they didn't want folks constructing impromptu handguns by cutting down common, unrestricted long guns.
The handgun registration/prohibition was completely removed, but the SBR/SBS/"Made from a..." portion was left in, apparently as an absurd oversight, and has been putting honest folks in jail ever since, with no logic to it at all.
When you read the law, and understand that ATF
must enforce the law as written, it's hard to see a way around it. The SCOTUS gave them an "out" in '92, but they chose not to take it, for reasons best left to speculation.