Everything has been killed with just about everything. WDM Bell killed a thousand and eleven elephant, mostly with the 7x57, and smaller but considerable numbers with the .318 Jefferies (which is actually a .330) and the 6.5x54 Mannlicher, which is only a little higher in velocity than the 6.5mm Carcano round. In all cases he used round-nosed solids. He also killed, if memory serves me correctly, sixteen Cape buffalo with that many shots from a .22 Savage Hi-power, which is no better than a heavy-bulleted .222 Remington. He didn't believe any animal deliberately charges the hunter immediately on being shot. But they will dash off in a random direction, unaware where the bullet came from, and will seize on a target of opportunity if the hunter happens to be there.
But Bell was a consummate anatomist, hunting in conditions where he often stalked very close, but could afford to pass up a dangerous opportunity, when he couldn't find an area the size of your palm. A logic I think he held, and which is worth considering, is that it would take some quite extreme sort of cannon to kill an elephant, or even temporarily immobilise it, with a shot just a few inches from a good one. He never used the .375 H&H, but admired it, and I believe the logic for rifles of that proportion is that a well-aimed second shot, at an animal just seconds away, may be worth more than more power in the first.
The leopard is indeed a very different proposition. Jim Corbett spent mostly nocturnal weeks hunting experienced maneaters, and often shot from feet rather than yards. He also found the .275, aka 7x57, a useful weapon, though for tiger he preferred a double .400 for occasions when he knew the shot would be very close. I've never shot either, and don't expect to. But I've examined a mounted leopard skeleton from close up. There was scarcely a bone you couldn't break between your fingers, but from about eight feet away it might as well have had no ribcage, for the skull blotted it out. Nobody should sacrifice the chance to shoot very quickly and accurately, once or twice, at one of those. I'd find and practice with a rifle which let me do what I once did for fun: shooting slow clays with a rifle, and trying to hit them more than a third of the time.
Back on the topic, a lot could undoubtedly be done in Africa with a .45-70, on game for which national restrictions allow it. It should preferably be a modern load, including bullet, such as would be usable in the Ruger No.1 - or better, anything that does the same with a repeating capacity.
I have a Winchester 1886 in .40-82, which I don't really expect to use on anything living. The recoil is quite comfortable, but I think the geometry of the conventional lever-action would inhibit the chances of a rapid second shot with the .405 in the 1895 Winchester. It has lived and shot for a long time since 1888, if Madis's serial number booklet is correct, and I can't see any parts which look like replacements. Nonetheless it does strike me as a less reassuring mechanism than a good bolt action, and I would want to be mighty sure no little bits of debris had found their way into the works.