JPK,
Glad to hear that you seem to be having a win with the chambering.

Don't get me wrong on this one.
If that rifle was mine the LAST thing I'd want to do is rechamber it to a flanged case.
I'd be busting my butt to get the gun working properly with the 458 cartridge.
Why?
To start with I hate changing the originality of a double unless there is NO alternative.
This is especially true with high quality guns like Marcel Thys.
I've been a fan of the 458 Win for a long time and have done a fair amount of shooting with that cartridge in bolt rifles.
I've always wanted a double in 458.
Came close a few years back.

I'd love to own a Thys 458 in the Holland style.

But......When the pressure is on, and a fast reload is required, I want to be able to just drop the rounds in, and shut the gun without worrying about any possible jamups.
This is a piece of piss with a flanged cartridge and even a poorly built gun can achieve it.
With a belted case, it may not happen quite so easily and the whole extractor/ejector system needs to be as slick as,
well, as slick as a very slick thing!