No, it will definitely not! Mannlicher-Schoenauer magazines are very cartridge specific. You would have to open up the receiver and to rework the cartridge carrier, if that's possible at all. On an 8x68 magazine the wall between cartridges and carrier spring is already quite thin, so you cannot remove any metal here. A Mannlicher-Schoenauer "Magnum" magazine limits cartridge over-all length to 86.5 mm =3.4", too short for any .404 factory load. There is no way to lengthen the magazine, as it is already opened up as much as possible by the factory. So, go for the 10.75x68, the 9.3x64 or forget it. One more obstacle: On all post-1952 M-Sch actions, including the Magnum ones, the cams that provide primary extraction ar machined as part of the barrel. Without the proper factory tooling, this makes rebarreling any post-52 M-Sch extremely difficult. I know a good gunsmith who did it once, but he will never try again.
See this thread also:
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=122479&an=0&page=2#Post122479