Honestly I can't see building a 404 unless going with a fairly classic big bore rifle build and that's going to be based on a 98 mauser or commercial variant and it won't be cheap or easy.
If on the other hand you simply want a 40 cal to launch 400 gr bullets 2200-2400 fps, I'd forget the 404 and go with a 416 Rem mag or 416 Ruger. You'll get exactly the same performance on game, but brass, dies and component bullets will be easier to find and less expensive.
I say this after having embarked on the 500 Jeffrey journey. The more exotic the chambering the more difficult and more expensive the package. If you're looking for a level of performance not available with more common chamberings or have a nostalgic itch to scratch, so be it. But if you're looking for a hunting rifle, a 40 cal 400 gr pill leaving the muzzle at 2400 fps is going to do the same thing to game no matter what parent action or what configuration your brass.
