Apart from cases being more difficult to obtain, why are you writing off the idea of a 10.75x68. It is much easier to set up a standard length action for this cartridge and despite what some are posting the .423 400gr at or near 2100fps is plenty for big DG. The early 404 loadings at this velocity accomplished everything required.
For some reason it seems that some think that the 40cals need to hit 2300 - 2400fps or higher to perform and supposedly get the penetration needed for big game and yet those same posters seem accepting of all the other larger calibres including the popular 458Win only needing to be doing 2000 - 2200 or thereabouts to perform. This just not stack up. Of course these velocities are the oft published ones. Many guns will not necessarily reach these velocities with factory ammo but still do the job. Remember too that most of the older advertised factory velocities were in 28" or 30" barrels which of course nobody uses these days or even much back then. The 425 WR got a little unpopular because of its unhandy 28" barrel.

My own Mauser 404 having had a long throat provided for monometals puts out 100fps less than factory advertised velocities (Parker Hale and Kynoch) but I did not notice buffalo getting their nuts in a twist about this.
The Type A Mauser was opened up to 404 before I got it but it sure would have been a sweet rifle to have now in the original 10.75x68 guise using modern bullets and powders.
Why does everyone think the .450/400 is a perfect cartridge (and it is) when it is easily duplicated by the 10.75x68 or 10.75x73 (404J) with a larger cross section bullet to boot?