The .404 is able to fit in a standrad Mauser, but you need a longer magazine and it will need a bit of work and cash at the gunsmiths side.

If i were to build a .404 i would use a 602 action and just barrel it .If it wasnt that i bought a good .M98 .404 yesteryear that was already converted and it was built by a good smith that had security in mind when he built it .