Don't confuse SBRs with Pistols - legally they are night and day different animals.
Personally, I think CalGuns is the place for this discussion - it's all already hashed out there anyway.
Ultimately what you are wanting todo is build something yourself from a 'DROSed virgin receiver' which has NEVER been a rifle; though you will have to DROS it as a 'long gun' because its not a complete firearm yet. Be sure that the paperwork is clearly documented on the 4473 that its a bare receiver. Also make sure that the FFL can document it is straight from the manufacturer without having ever been 'made into a rifle'.
Alternatively find a place to buy what you want as a 'dimensionally compliant single shot pistol' FYI this is already how some people are bringing off-roster firearms into CA.