This has been a most enjoyable thread to learn from. Thanks all!
The old "bell shaped curve" being what it is, some responses were more passionate than others, while some were more knowledgeable than others. Without intending to gore anybody's ox here, I'd like to add that unless a pre 1946 Mauser has been custom re heat treated, irrespective of its length (Kurz, Standard, or Magnum) it was originally differentially heat treated, which resulted in variable C and B scale readings all over the action. An example: harder bolt lugs than bolt lug recesses. This variability, however, is not a flaw - it's a design element. With differential heat treating, the action was meant to stretch, rather than shatter, when pressure exceeded a certain point. So the classic long Oberndorf Magnum action truly was not one whit stronger than a Standard Modell K98. Just longer. Should you re heat treat your carefully selected surplus action if you're intent on stuffing a .404 Jeffery into it? That's between you and your smith, but because of the original heat treat design, its my understanding (from an engineering buddy in the business) that its damn hard to even measure the hardness in the right spots to see if you should. All in all, if you're using a DWM, Oberndorf, Mexican 1910 for your critter gitter (talking in general here, not .404) I do think its a good idea. For this and other reasons though, I've stopped using Oberndorf and DWM M98s for any of my builds, instead shifting to their Czechoslovakian M98 cousin, manufactured up through 1946. If you have two equally well made original actions, one Oberndorf or DWM and one CZ, you'll immediately note the greater slipperiness (lubricity?) of the CZ over the "hitch and git along" feel you'll get when cycling the others. I prefer not to use M70s pre dating the more modern ones due to poor gas handling if things go sour. Yes, I have a prewar in 300 H&H (four down!) and it is smooth, but I like my right eye a lot. The esthetics of some of the other possible action choices aren't, to that right eye, very classic looking and many other perfectly good choices are simply too danged expensive for this fixed income viejo.
So summing up, it's been demonstrated over and over that you can house the .404 Jeffery in perfect safety in a standard length M98 when opened up by a careful workman. Most of the metal removed should, to me, come out of the rear of the action rather than the front, though Ken Waters once wrote in Rifle magazine that the notch in the rear of the receiver ring of Browning .375 and .300 H&Hs, along with reduced size of the bottom locking lug recess really bugged him, but added that he'd never heard of it resulting in a problem. Beyond that, reasons of ease of conversion or cost of build are just one man's way of exercising his ability to choose, and to bring his own ideas to fruition (or not). It's my money and my rifle, and I'll do as I see fit; it'll be safe when I'm done with it, thank you much. And I'm behind you doing the same with your money and your rifle, too.
I think I hear a .500 Jeffery calling me. I've got this VZ 24 in fine shape that hid out down in Brazil for years; it shows little wear and needs to do something besides take up space in the back of the safe....