If the receiver was stripped from the factory and was ran as an "other" on the 4473, you can build an AR pistol no problem.
If it ever had a buttstock on the receiver, legally it can't be a pistol and you'd have to SBR the receiver if you wanted a barrel over 16" OAL.
If it was a virgin receiver or started life with a pistol extension, it can be converted to a long gun and pistol, back and forth, freely.
Form 5 is a NFA form that you shouldn't worry about, this is a standard pistol
Are you sure about the bolded part above? I thought once you went to a long gun (stock) you couldn't go back to a pistol.... I could completely be wrong.