Hey Mike, sure, a Mauser is nice. I was just recommending the easiest and cheapest way. Case water capacities are the same, so the 416 Rem is not a bigger case.

Building on a Mauser, and doing a nice job, cost money. If the owner has the bucks, then go for it. I am building a couple on Mauser actions for my personal collection.

Mafia, you need a post 64 classic push feed (the new one) M70 action. Ideal is a 375 H&H length. You likely will not find a 375 H&H length unless you spend a bunch of money and buy a new one, then any standard long action Classic CRF M70 will do. With a standard long action , you will need to install the longer magazine box and follower, machine the rear bridge, re-drill and tap the rear bridge for .330" hole space rear scope mount, shorten the ejector and bolt stop. And open the bolt face if it is not already a magnum. Pay attention to the items in my previous posts.

Be very, very careful in barrel selection. No one but Krieger makes a proper .403 bore/.411 groove barrel, as far as I have found. I would not use a button rifled barrel, even if you find someone to make it. You need 12 inch twist. There are a few other cut rifled barrel makers that may take this project on, but I do not feel the quality of their barrels is up to a Krieger cut rifled.

Who to build the rifle? Big question. You need to find someone with experience with the 400 H&H cartridge, and who thoroughly understands big bore rifle construction. I have reamers, dies, all the specs, load data and performance, but I do not know of anyone else equipped with the tooling and information.