Dok:
To my knowledge, the only gunsmith in Australia who has made SxS double-rifles from shotguns on anything like a regular basis is Rolph Bachnick, I think his Melbourne shingle says 'Safari Arms'. Dunno whether he will still do this though. I have one of his efforts, a .577 Light Nitro double on a Greener Empire action. I have posted the story of that gun before: newbies can read it here.

Ron Webb made a couple of U/Os on the Ruger Red Label action years ago, chambered in .375 Flanged Magnum. I'm told they were good, but I believe Ron is long retired.

Ellis Brown favours German shotguns with a cross-bolt third fastener, however my advice fwiw would be to forget the .416 Rigby and the .470 Nitro, they are fat cartridges and wouldn't leave much meat around the chamber. Back-thrust on the standing breech can be quite high with full-nitro loads in fat cartridges. Slimmer cartridges or lower pressure options should be selected IMO, despite Ellis Brown's enthusiasm!

Remember, you are dealing with shotgun actions proved for service pressures in the 3 to 5 ton range, and proposing to fit them up for 14 to 18-ton loads!!!

While the pressure calculations are much more complicated than that, of course, I hope it still offers some sobriety to your proposal!