This topic was rehashed about a week ago and I learned something new (again)!
First let me say it is not correct to list a stripped lower as either rifle or pistol on a 4473. If the FFL knows what he's doing he won't do either. Up until one of the recent form revisions (2011, 2012...???) the correct listing for a stripped lower was "Other". You list it based on what it is when you take it home, not how you intend to use it. The 4473 has been revised and now has category "Receiver"; that is the correct listing for a stripped lower. You can then build it out however you want, however "once a rifle always a rifle" still applies. In other words you cannot build it as a rifle and later convert it to a pistol - that is a definite BATF no-no. You can however convert a pistol-built lower to a rifle whenever you want, as long as all other "rules" are followed i.e. Form 1 for an SBR.
29.
Type (pistol, revolver, rifle,
shotgun, receiver, frame,
etc.) (See instructions for
question 29)