1. Buy the SBR you want with a "pistol" buffer tube. Replace buffer tube with proper stock assembly when paperwork is approved.
2. Buy a rifle configured (except for barrel length) as the SBR you want. When paperwork is approved pull the barrel, cut, crown, thread to length.
Either is better, IMO, than having a gunsmith build your SBR and then watch as the government forces him to store it until they get off their fat useless asses and process the transfer.
1. Buy pistol lower
2. Buy upper in caliber/length of barrel you desire
3. Spend $200 on ammo
Then thumb your nose at the man when you have what is essentially an SBR without the requirement of stamp, cost, and wait. Plus you're $200 ahead on ammo.
Just be careful with the whole constructive intent thing if you go the pistol route. I've heard even having a spare stock and non-pistol buffer tube lying around is enough to get you charged. Granted this all could be conjecture, but I'd definitely check with the ATF. If this was true you would think they could nail you as well if you just had a complete non-SBR build lying around since it takes all of 30 seconds to put that lower on a pistol upper creating a non-registered SBR. Really wish we could at least get rid of the SBR and silencer stuff from NFA but ya fat chance of that happening.
Take it easy there. Having the parts assembled in a legal configuration along with the parts necessary to assemble in an illegal configuration does not constitute constructive intent. If it did, then anybody with a rifle or shotgun and a hacksaw could be charged with constructive intent to assemble sbr/sbs. The action of the individual is what matters. A pistol can be built using a carbine buffer tube and leaving the stock off of the final build. As long as there's no witness marks from installation of a stock on that buffer tube, there's nothing to worry about - even then, the burden of proof is still on the prosecution to prove that a stock was installed at the same time as a sub 16" upper. Now, if you have no pistolor sbr lower and one or more sub 16" uppers with evidence of firing wear, you might got some 'splaining to do.
As long as it's built correctly, there's nothing wrong and nothing to fear.