Just a couple of comments to keep things rolling.

While JJ may be able to get a rechamber in the Thys books because of their relationship (close)unless he has made a Liege Proof House stamp the rifle will be unproofed.

Marcel, who built the rifle in the mid 80s, is not a hunter. Most of the famous gun making companies were run by non hunters and the gunmakers almost never hunted. They relied on their customers to tell them what to build.

The 458 headspaces on the very slight shoulder. There is no bolt head to extract it if it sticks. there is nothing wrong with it in a proper action but a Double is not it.

I think you did good getting it with the idea of converting it to a 4502 or a 450 3 1/4. It will probably be fine as a 458 but I am a firm believer that Murphy was an Optimist.