BATFE requires that it be affixed in such a way that removal cannot be accomplished with common tools. Blind pin is the best way.
Also, be aware that once you SBR a pistol, it is now legally a RIFLE. Once you do so, if you revert it to the pistol configuration, you now have a "Pistol made from a rifle", AKA a big no-no. If you want a pistol, leave it as is. if you want an SBR, do that.
Personally, I would wait for the tax stamp, then aquire the stock (to avoid the constructive intent issue), rather than gunsmithing your pistol to a 16" barreled rifle, then gunsmithing it back once you get your stamp.