I just bought a rebored.404 Jeffery barrel that has large ring Mauser threads. I have a spare German-made 1909 Argentine Mauser action in great condition. What alterations are needed to the Argentine action to feed and shoot .404 Jeffery? Will the bottom metal have to be replaced?

Replace the 1909 receiver with a VZ24 receiver. You'll be way ahead of the game.

I will sell you a VZ-24 action. You will be well advised to get bottom metal specifically from Duane Wiebe. It would cost you a lot more to try and re-work your present bottom metal.

Here is a pic. of how I modify a Mauser receiver.
It's a good solid setup. Receiver is fixtured upside down in a ported aluminum bar.
Under cnc control, 2 axis proto-trak. Basic, but gets in done, it's accurate and they work.

The one in pic. is actually 404 built on an FN,
For the New Guy at Heym, Weibe bottom metal was
used. Additional rail and feed ramp work was required to attain perfect feed and funtion.

A few years back Sarco had un-finished FN recievers, New picked some up and we built them out. pac-met did the heat on those 35 RC as I recall, a 300 Win and the 404. The Sarco fn receivers where dead soft when purchased from Sarco.

Condition is everything on the 09 receiver.
If they a show set back they obviously could not even handle the 7.65 argentine, While some 09 example show no set back, better heat treat on some but not others? I tend to think so.

I have found that cleaning up 09 damaged lug seats possible and usually .007 to .010 cleans them back up. Then pac-met for the Burgess method and it's good.

Timan/Stuart Satterlee/Satterlee Arms

Same thing but without the fancy jig and cnc. Just scibe and follow the line. Of course alot slower.
Don