As far as switching over to an SBR, the Legal ruleing from the ATF vs TC Contender case is that once a pistol, always a pistol. A pistol can be made into a rifle but not vise versa.
A Pistol can be fitted with a stock after the barrel is switched to +16' but a rifle can't be made into a pistol, otherwise it's an SBR.
You can't fit a Romy/AMD type front hand grip to the weapon while it's still in pistol mode or it's an AOW.
Since the AK barrel is fixed, You must file the SBR paperwork and pay the tax before fitting a stock.
You must file the AOW paperwork and pay the tax before fitting a Front Pistol grip sans stock.
To answer the follow up question, Yes you can turn a Ak pistol receiver into a +16' BBL rifle but it would always be legaly considered a pistol and all the local and FED laws for selling would apply.
On a side note. It looks like Atlantic is useing the same Romy FSB/GB that K-Var has in it's parts section.
I like it.