Ducmarc,
Here is something you can do to decide if you really want to risk rechambering a Russian Baikal to a 45-120. Find a used one for sale and ask to measure the barrel diameters back in front of the breech. If you ask enough sellers you will eventually find one who will pull off the forearm for you to do the measuring. Be sure to take along a dial caliper and a pencil and paper with you to write down the numbers.

Next, subtract the bullet diameter from each barrel diameter and divide by two and that will give you the barrel wall thickness at the breech end of each barrel. Then you need to consider that you are going to be moving the case mouths forward by over an inch, this may put them into and area where the barrels are even thinner.

Finally, you need to be honest with yourself and ask yourself if you really want to risk death and dismemberment just so that you can stuff a longer cartridge into a gun that may already be near the limits of what it can take. If you need more power, buy a gun that will take longer shells, but I would not recommend hotrodding a 45-70 in a Russian Baikal too much.

Don't misunderstand my intent here, I have nothing against the Baikal's, in fact I own a 30-06 over under made by them. Whatever they are using for barrel steel is really strong and tough stuff. That being said, I wouldn't want to push it without knowing what they say about the upper limits of what they can stand. Possibly a dealer could get that information for you, but I doubt that they will tell you what kind of steel they use or what the expected limits of it are. Be Safe and Best Wishes.
Bob