AFAIK, the m39 when re dated, even on an antique rcvr were 'new guns'
And old m91 antique is just that.. and antique.
I can't help you any on the legalities of who can or can't own an antique arm.. ( too many state rules and then federal codes, and while I have an idea.. .. I'll leave it up to someone else who knows for sure ).
I believe you cannot make it into an SBR for a couple reasons, and again.. I may be wrong, but here is why I think you cannot do it without a form 1? SBR
1, I know C&R guns loose their C&R status if they are modified from original condition ( as issued ), if that modification was not also done ( 50ys prev ).. that might make me think the same on an antique loosing it's status.
2, also.. it fires obtainable fixed cartridge ammo.. I DO believe you could get away with something that would otherwise be NFA, if it used a primitive ignition system, like a muzzle loading flintlock/percussion gun.
That and 4$ will get you expensive coffee at starbucks, soo.. sorry.. no help. I simply don't think it will work, but yet don't know all the rules and regs to quote you ..