From what I can see from the drawings and photos of the Baikial 45-70 double rifle (marked in the states as spartan?) It is built on their 12 gauge s/s action. If you contact the importers they might be able to get you a bare barrel set that will just fall into place on your action. In my experience with Biaikals they are rough as a badgers arse but the critical tollerences are very good, because they do almost no hand fitting. You could run a 45-100 reamer in at your own risk. IMHO this will work out cheeper and they will extract/eject. I personally would not take the effort to bush the firing pins for 45-70 or 100 as they are not high pressure rounds. Might polish the pin tips to make sure there are no burrs and leave it at that.
They have a rep for being as tough as hell, I don't know anybody that has broken a Biaikal and hundreds are used here as real knock about keepers guns, never cleaned, oiled etc.

Regards