You can convert a standard length Mauser to 404, (or even 416 Rigby) is price is no object, but it better not be any object at all.
If price IS an object you are far better off to just buy a commercial rifle in 416 Remington, Rigby or Weatherby.
I have done Mauser conversions to both the 416 Rigby and 404 Jeffery, and the time and parts involved are costly. Then you have to have a stock hand made, as it's no longer a "standard" form.
So by the time you have one built you'd be into the project several thousand dollars
You can buy a Ruger for about $1600, and a Remington for less. You can get a CZ for about the same I think.

If you want a standard length Mauser in a "40" you might want to re-consider reloading and have one done in 416 Taylor. A full reloading set-up would be very cheap in compairison.