You're going need much more modified than that... You'll need a custom magazine box, follower & spring, you'll probably have to have the feed rails opened up, as well. Much more involved than just opening up the bolt face when going from a .308 action to a .404 Jeffery/RUM cartridge width spec'd action.
You would be easier (and cheaper) to start with a Rem 700 SAUM/WSM action, or a custom aftermarket action already machined for WSM sized cartridges.