I hate to say it, but the 416 Ruger is probably the logical DG cartridge for a standard Mauser.
Opened up to .423" would make a modernised 404.
In fact, I have a super clean 10.75X68 DSB that I could do just that with! (calm down guys, it has been restocked already).

I agree with the OP about classic Mausers and nobody could be faulted for going with Joel for the work.
Funny thing is, classic Oberndorf Mausers can still be found at somewhat reasonable prices. I had my 10.75 on the market at $4.5K. Not a peep. A SB Kurz at $3.5K. Not a peep. Sold my Mahillon 416 and my Gibbs .280, both on SB magnums but it took forever and they went for low enough that I should have just kept them...
One thing about the original magnums that is different from the newer ones is that Mauser offered bottom metal designed specifically for the cartridge. I think maybe four or more variations IIRC. That was the thing about my Gibbs, the magazine was so tapered to fit the 280Ross that there wasn't much else that would fit well in the magazine and the rails were already widened to feed the 280 which has quite a big base/rim diameter which is even bigger than the belt on the 375H&H so I am not sure that the 375 was even much of an option.