10.75X68 would be my choice - or a .416 Ruger, or a .416 on the .376 Styer case - or a .416 Taylor.

I'm too old and beat up to shoot the .45's now.

Since you are looking at .416's and .45's, I kept my responce to those.

I would personally just as soon take my 9.3x62 or if laws required .375 or larger, my .375/06IMP. It will drive 300gr. at almost 2,500fps and 270's at 2,700fps, both 100fps higher than original .375H&H ballistics of 1912. One can always slow them down that 100fps for lower pressure and still have real power and flatter trajectory for the plains game than the .416's or .450's can deliver.

Being easier to shoot accurately, pin-point accuracy makes up for any loss in perceived 'power'.