Allow me to play devil's advocate.
You will have a bit of remnant .458 WinMag throat if you re-chamber a .458 WinMag barrel,
even with the .460 G&A reamer from Manson, unless you set the barrel back, and turn a 24" barrel into a shorter barrel.
Not a bad thing, either way.
The .300 RUM follower and windowed box that came in the factory M70 rifles should be the cat's meow for 3 down in the box.
Or try the Sunny Hill drop bottom metal for M70 .375 H&H and experiment with the followers and feed job.
I see the .460 G&A as a welcome way to re-barrel or rebore an unwanted .375/404 Jeffery or .410/404 Jeffery
to make a more interesting rifle.
The .458 is tops.
