Christopher,
Well, this is turning into another one of those you need bigger as that is always better topics.
Onfortunately, no one has considered teh North American aspect of your question. Certainly this rifle should be designed for African big game but to be honest, it will probably be used much more here at home so I feel you should choose a round that will work for both very well.
Now the big 458s and 475s and 500s are great and if you need to stop a foght with a ****ed off cape buffalo or elephant, bigger is generally better if you can shoot the cannons.
That said, most bad situations are a result of either poor shot placement or poor bullet performance. A PH would much rather see their hunters carrying a 375 H&H or a traditionally loaded 416 Rigby then a 378 Wby or 416 Wby and certainly a 460 class round. Simply because they realize that its that first shot that can either end the fight very quickly or result in a very serious situation.
Every cartridge you list is adiquate for heavy game in Africa even without the 458 caliber rounds and larger.
Hands down the most useful over the widest range of hunting would be the fast 375 rounds. With proper bullets they will cleanly harvest the largest game with a properly placed bullet.
THey offer much more ranging ability then the larger bores and they are much better suited for North American game. Still, if your going to build a rifle for use on heavy or dangerous African game I would simply recommend the 416 Rigby as it will do anything you need it to and have plenty of power as well as boing easy to shoot which means you will actually shoot it more and become better with it.
You can also load it with 350 gr bullets for use in North America and have a big bear rifle with no peers. They work very well on elk and moose as well.
While you could use the 458s here at home, they are far out performed by the smaller bores in trajectory which can be an issue here at home.
For an all around heavy game rifle in Africa and here at home, the 416 Rigby would be a very hard round to beat. The 416 Wby is also in this same class but velocity should be dropped to around 2500 fps with a 400 gr bullets for better bullet performance.
If you like the 416 Rigby can also easily be loaded up to 2700 fps with a 400 gr bullet just like the Wby in a modern rifle. It is a much more friendly round loaded to 2400-2500 fps though.
Just my opinion, leave the true big bores and get one of these 416s. It will serve you well.
Also, ammo availability is very good in Africa.
Good Shooting!!
Kirby Allen(50)