If it gets checked, my bet is that they call it a firearm until proven otherwise.
If the frame letters to be of one of the excluded calibers, that would make it that much easier.
A letter (on letterhead, from an antique dealer) declaring that the frame is from an antique firearm is a remote second best.
The safest and easiest route would be to import it as a frame, with all the registrations in place, then have it built up as a (eg: .41 cal), and have it "un"registered.
Since Customs, at times, seems to have their own ideas about what is and is not considered a firearm, it amounts to a crapshoot on your wallet.
And god alone will be in a position to help you if it is brought into the country as an antique, and is subsequently built up as a non-antique caliber (ie:45LC, 38-40, etc) using readily available parts.
Cheers
Trev