What do you actually want to do with it?

If it's just a fun project, I would try a calibre other than one of the classic big game cartridges and make up something like this:

https://www.outdoorlife.com/blogs/gu...re-cartridges/

A cartridge like a .44 magnum or maybe even something like 444 Marlin or 45-70 would be my first choice as pressure is lower than that of the typical big bore rifle cartridges. Select your steel correctly and the insert should be able to handle the pressure without any assistance from the barrel. I would make the insert not more than about 12 inches long and wouldn't even bother to try and rifle it. The result would be a cheap, fun conversion that will make big holes in paper and knock over steel plates with authority, be usable for eg hunting pigs at close range and is reversible merely by taking out the insert. It would be necessary to look into the licencing aspect as I expect SAPS would regard the insert as a barrel, hence requires a licence. In any case you'd need a licence to possess the ammo unless you already have another gun in that calibre.

If, on the other hand you are looking for a "real" big bore rifle to eg hunt buffalo with, it will become a much bigger, more complicated & expensive project requiring a lot of paperwork that CFR would take ages to process and I think it would be more sensible to just buy a big bore rifle. If you want a break barrel, Drillings come up on the Classic Arms auctions from time to time and generally don't attract much interest.