The 404 is a very classic big bore in a standard length rifle. The largest classic British round that can be safely shoehorned into a standard action is the 500 Jeffrey. Duane Wiebe sells three and four round drop box magazines for them. Neither requires a long trigger. He also sells 404 Jeffery magazines. Conversions to either round requires the talents of a very skilled Mausersmith! The special magazines and the work to convert the actions is not cheap. Conversions to 404, done the way most very early Jeffery rifles were made, can cost much less, but they are rather crude. Pac-Nor barrels has a barrel print taken from an original 1905 vintage 404 rifle. They make a fine barrel, as well. Here is Wiebe's website.:

www.customgunandrifle.com

www.customgunandrifle.com/products/bottommetal/mauser-bottommetal

Give Duane a call and discuss these conversions. This will help you decide if it's worth the expense and effort.

BTW, the 416 Rigby, in a 3.6" magazine, is considered safe, but you would be limited to using only Federal and Hornady factory loads. I own a barreled action, made in such a way, using a modified Wiebe 404 drop box magazine. Some of the old Kynoch loads are as long as 3.75". Rigby made several standard action 416's in the first 15 years or so after WWII. Some of them are said to have developed setback in the locking lug recesses, as too much metal was removed from the feed ramp and lug recess support. They, no doubt, were opened up for the use of the longer rounds, as long ogive bullets were all that were available back then. The advent of the 3.6" 416 Rem and the 3.34" 416 Ruger caused the American bullet makers to shorten 416 bullet ogives considerably.

Good luck!