I think the .375H&H is the ideal cartridge, but a multi-cartridge bolt action rifle is the ideal rifle, so of your 2 options, I'd get the .458 Lott

If that's the rifle I'm thinking of it comes in .375H&H too.

I second the talking to your guide/outfitter, there's a chance he'll be screaming at you not to have a single shot...if so that would answer that I think.