The stocked bare receiver is supposed to be listed as "other" on the 4473, But since it has a stock on it from the factory it is considered a long gun by the BATFE. so taking the stock off and using the receiver as a pistol frame would be considered making a short barreled rifle by the BATFE.
Under the instructions for that part of the form it says
If a frame or receiver can only be made into a long gun , it is
still a frame or receiver not a handgun or long gun.
It started life with a stock so it has to keep the stock. If the stock was not installed from the factory or dealer you would be good to use it for anything