Funny, I just came into the NFA forum to post about my lust for an Obrez. I'm starting to think that the chief in Worcester won't approve a Form 1, so I'm looking into other options.
Meanwhile, I realize you could do it without involving the feds with an antique Mosin, but I'd feel awful ruining one. I was thinking more along the lines of a Type 53 because they're beat to **** anyways.
Just use the antique receiver and a crap barrel. Of course, pulling the barrel might take more effort than scratchbuilding a new receiver.