BC, I already load for the .416 in new cases. I just happen to have a bunch of .375 H&H brass, and no longer a rifle chambered for it. I thought about forming it and using it for midrange lead gas-checked bullets for practice in the .416. I use a Wilson case trimmer, and would definitely square and chamfer the case mouths before any accuracy testing, but have never done neck reaming, not finding it necessary.
I took a bison a couple years ago with the Speer 350 gr semi-spitzer at 2600 fps. DRT.