Hi Ckhobart,
Welcome to the world of building double rifles. I don't want to come off as a "know it all", but I think you have strayed off in the wrong direction from the beginning. Don't feel bad , it happens to many of us. I am going to give you some free advice so take it for what it is worth. First of all, do not buy a Biakal with the idea of converting it to another caliber, that is not practical or even safe for many reasons, not the least of which is the size of their barrels back at the breech.
I bought one of their 30-06 over and under models just to try and figure out how they could get by making such small diameter barrels. My 30-06 has barrels at the monoblock that measure only .765 inches and they rapidly decrease down to around .600 inches across. These small barrels are not conducive to being replaced with anything other than some caliber that would be made smaller and weaker. I don't own one of their 45/70 guns, but if they are made in this same fashion, they would not be a good candidate to convert either. I see on here all the time about someone wanting to rechamber their Biakal to some bigger and longer .45 caliber cartridge. My advice, Don't Do It!, Don't Even Think About Doing It! There is not enough steel in their barrels to warrent even thinking about making them into something larger, plus the fact that we don't even know what the Russians used for steel in them.
Don't misunderstand me, I am not running down the Biakal guns, they are dandy as a entry level double rifles, cheap and well built as far as their strength goes, but they are also somewhat crudely made. They have cut corners where they safely could. Even if you could convert one of them to something else, you would have to do a massive amount of work to make it look presentable as a double rifle. For one thing it would have to be restocked in order to fit anyone except somebody with the largest hands,(the pistol grip is way too long on all of them to make it comfortable to shoot the front trigger). The stocks are crude in other ways as well. They leave so much extra wood on them that they look like they were made by a highschool boy in Junior High shop class.
The next gun you talked about converting was a cheap, older hammer gun. This is not a very wise choice either. Depending on how the firing pins are set up, it may be unsafe with even shotgun shells in it. Most hammer guns are not really good candidates for converting because of their age and the lack of any kind of third fastener. I have only seen a few hammer guns with a rib extension, and have never seen one with the Purdy sliding bolt. What you should be looking for is a European 12 gauge with or without a cheek piece, that has double underlugs and a third fastener of some kind. 12 gauges are much easier to convert than a 20 or even a 16 gauge because they have so much more metal in them and have more room to work inside the monoblock.
If you buy a 12 gauge European shotgun, with possibly bad barrels or something, your donor gun may already have such desireable features as firing pin bushings, sideclips, a cheek piece, and sling swivels, plus you can buy it at a bargin price. Rememeber, you are going to replace the barrels anyway. If the wood is in decent shape you can get by re-usung the stock and forend. This all saves time and money and gets you shooting it quicker. You can buy these for very reasonable prices if you watch the auction sites and you will end up with a much stronger and better donor gun.
Bob