I was going to mention the cost different of the .416 rigby and .375H&H, I remember seeing factory loads for the riby around $200/box I wanna say, but those were federals I think, I think hornady may load some cheaper, like 100/box, I've seen 375 for 70-90/box, not sure what currently they're going for though.
If you get a .458, whatever you do get the .458Lott and not Win.Mag! The lott can shoot the win.mag, and you can load the lott down to I believe even .45-70 levels, but you already have a .45-70, which is perfectly fine for bear, just not sure what kind of range you'd get.
I saw Nugent shoot a elephant inthe head with the 375H&H, though I hear its small for elephant, its certainly a worldwild dangerous game round.