From cross referencing load data with different manufacturer figures I think the flattest performance will come from a bullet of around 350gr. Lighter and it's like throwing feathers, heavier and the velocity isn't there. I will look into the bullets mentioned, although there's a chance they may not be available to me here.
It's good to hear this is being done. I like to mess about with the boundaries of what a gun can be used for. They all have their designed application which they are best for but they can also usually be tuned to work for other tasks with a change of powder and bullet. In the UK a lot of my getting used to the rifle will hopefully be with light .45-70 bullets and low charges. It won't do anything for recoil familiarity but what it will do is make me familiar with the functions of the gun - cycling the bolt and the safety catch, trigger pull etc.
I was thinking irons + maybe a low mag scope for the big stuff and something like an 8x56 Schmidt zeroed for the light bullets. Two sets of quick release mounts will allow either to be fitted quickly and be zeroed differently. That way the irons are there for what they are intended - whacking large, close targets that need the gun to be very responsive. The large scope could be added when a smaller animal needs to be aimed at further out. Close up the difference between the two won't be far enough apart for the irons not to function to an acceptable level with the light bullets too. Sure it's not perfect, but it will work. Dropping rounds and reloading won't take long, neither will adding or removing a scope. Anywhere I think I'm going to get a bit close to anything nasty the gun can be kept with irons and loaded with 500gr something with 4 in the mag. See something lighter further out - put on the 8x scope and pop a couple of light bullets on top of the heavies. With a suitable belt pouch with good access that will take what, 15 seconds at the most? If the game has legged it in that time I will be very unlucky, and the rifle will be carried in a defensive form for the whole hunt. Stumbling upon a buffalo doesn't allow time for changes to be made but sneaking up on a long shot usually does.
I had to look up the .458 B&M. Never heard of it! It looks like a handy cartridge though.