I have done this in the past. I used London Guns bottom metal, but there are out of business now.
It still took a great deal of work. The mag walls have to be thinned some to make room for the cartridges, and of course the action has to be opened up too. I would set the magazine back by milling away the "hump" at the rear of the action where the Military magazine box would bear. (the very rearmost part where steel meets steel) and set the recess of the front boss of the trigger guard forward to accept the smaller boss around the front guard screw. Then I would cut away the inside of the box ends to make them thinner. Doing all that gives you much more room to deal with.
next I would lap the bolt back until the 3rd "safety lug" becomes a bearing lug. At that point the reduction in strength you have in the lower abutment that was taken away from cutting the steel away is more then compensated for my adding a 3rd lug that's fully bearing.
After the work is all done and the whole job is polished and tuned for super smooth feeding, I would send the bolts and actions in to Industrial Heat Treat in Salt Lake City for Heat Treatment.
When you get your action back, it's got to be polished on the insides of the ways, and the bolt lugs and body, but that's fast and easy.
When it's all done you have something to rival anything that came out of London in the 20s.
The 404 is originally loaded to only the same pressure as a 303. The Standard Mauser (Gew Length) is more than strong enough if the works done correctly. I load mine up to 2350 FPS with 400 grain bullets. Same as the 416 Rigby. Works like magic. I LOVE my 404.
To get brass you can but Norma (expensive) or you can re-form 375 Ultra Mag. The rims are a little "too small" but I have not found it to be a problem.
I have heard tale that Hornady was going to bring out the ammo too, but i have no idea if that's true or not. They would be well advised to bring out 404 "basic brass" (cylindrical) as thousands of wildcatters would line up to buy it.
Give them a call and ask. They do listen to the public.
SZ