Thanks to Sir Ron, I've now converted my New Haven M70 to a .458WM+. I've been wanting to do this for a long time, and with Sir Ron's coaching on this thread, I got it completed. Also, previous to this and years ago, in correspondence with Sir Bob, I wondered about getting a .458 Lott, and his response was to instead, get a .458 WM and load long since it had a better throat. Standing on the shoulders of giants...
This is the new magazine box:
After popping out the rear spacer with a wood chisel, it looks like this compared to the original box:
The new, shorter ejector compared to the original, longer ejector:
The new, shorter bolt stop compared to the original, longer bolt stop.
Finally, look at how much more space is now available! For common reference, the cartridges are vintage Remington 510 SP ammo.