Usually stripped lowers are classified as "other", not "pistol" or "rifle" by the FFL on the federal form when you buy the lower. It's been that way for me for every stripped lower without my saying a word - the FFL checked off that box without asking. That should allow you to build it as a pistol or rifle, assuming it's legal in your community or state. I'm only going by what I've heard since I've never built or had any interest in an AR pistol. But again all my lowers were checked off as "other" then I built them as rifles. If the FFL checked "rifle" I don't think you can legally convert that lower into a pistol.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek