I understand your pursuit of the obscure. And I don’t hate 425WR, if you had a Westley Rifle, I’d be willing to suffer its limitations and difficulty in finding good brass, good bullets, etc.
Since you’re looking for something off the beaten path and you want a 400 class gun, have you considered a 450 rigby? It’s a 416 rigby necked up to 458. Plenty of bullets, plenty of brass, plenty of power. It’s a new caliber, only about 20 years old, and to my knowledge only factory manufactured in Rigby rifles. You‘d love the power and reliable feed if it will fit in the action you’re thinking of using.
Other new arrivals would be the 400H&H that is a necked up 375HH and brass is super easy to form and it has plenty of punch.
Down the memory lane but also cool is the 400 Whelen which I suspect could be made from 30-06 brass in many steps, or from 35 whelen in fewer steps. This gets you the 400 caliber, plenty of punch, and a bit less difficulties with brass.
Finding brass isn’t a trivial consideration these days either. Some of these esoteric vintage calibers had Bell brass, Bertram, Jamieson, and Qualcart. Most are no longer made, many do re-supply orders once every 5-7 years so when you find it, you must hoard it. With the examples I listed above, there is no world crisis that would deny you the ability to get brass or bullets as we can scrounge for 30-06, 375hh, and 416 rigby brass to use for forming pretty easy, same for the bullet diameters they use.
Beware the uniqueness of the .435” bullets of the WR though. Hawke bullets are notoriously soft and that really leaves you with two SKUs made by Woodleigh. God forbid something happens to Woodleigh, the gun has no alternatives in a soft or a solid. With the others, there are many, many options.
Just some alternatives and considerations to keep in mind for your build.