before you submit your paperwork on your SBR - as noted above build it (and register if your state requires such) as a pistol.
once your build is functional, then and ONLY then if you still want to make a SBR out of it should you submit your paperwork.
logic behind this method is twofold
1) it allows you to legally purchase, build, posses, and test your new firearm (with everything except a stock) before you make unchangeable decisions about the serial # of the lower you're going to be using.
additionally if you dont build it as a pistol first, and dont already own an AR-pistol lower or another SBR AND have a functional ar-rifle... having a sub 16" barrel (or built upper with a sub 16" barrel installed) on hand puts you in the grey area called constructive intent of being in posession of an unlicensed SBR.
I'd cry if i submitted for a form1 build of a SBR and once i got my stamp back found out my lower was somehow out of spec and i had to re-do it all over again and wait another X number of months for a new stamp approval.
2) its perfectly legal to convert a pistol to a SBR, the only difference is a receiver extension that can be compatible with having a stock on it or not and the necessary NFA engraving once you're stamp is approved. basic Pistol receiver extensions can be had for under $20 and utilizes a standard carbine spring and any standard buffer you would use in a normal 6 position stock assembly.
so once you get your stamp and get your engraving done, the only change necessary is to unscrew the pistol receiver extension, install the 6 position receiver extension and put your stock of choice on it. and if you're in a state with a handgun registration requirement, file any necessary paperwork to remove your formerly handgun from said registry.
needless to say its a cheap way to not only work any bugs out of your gun before you make long term decisions as to its legal status, the extra $20 or so you spend on a pistol receiver extension is the absolute cheapest legal shield you can buy to protect yourself while you wait for your stamp to be approved.
and hey.. now that you've got a pistol receiver extension on hand once you convert... well gee.. shame to have spare parts around.. you now need to build a pistol too!