Don't take this as gospel, but yes I think you can do whatever in the world you want with ML rifles like that, since they are not firearms, EXCEPT for:
-Savage 10MLs, because they are smokeless powder-capable, and
-Any rifle system that is interchangeable with metallic-cartridge rifle barrels, like T/C Contender & Encore, NEF Handi/ Huntsman, Rossis, CVA Optima Pro, etc.
Hmm, this brings up an interesting question - say I have a pre-1891 "non-firearm" like an early Mosin or Commission Rifle. Could I make that into an 'SBR' and not have it be considered an 'SBR'? Vewwy Vewwy Intewesting.