How about a Baikal 45/70 double? Just tossing it out for giggles :-)
I'm not sure why a .458 cal, 400gr bullet of decent BC moving at over 2000fps, that produces over 4000ft/lbs of energy is deemed inadequate or problematic on buffalo. Not like it hasnt been done before. Is it the best choice? No. Is it adequate? Of course. There seems to be an ingrained prejudice towards the .45-70, perhaps due to those who try to attribute to it qualities it truly lacks. However given the ballistics above there is no reason it should be considered inadequate at typical buffalo ranges.
I doubt that the Biakal will handle the pressure of 2000 FPS with a 400 for very long. There are much better rifles that will handle that.
If you can $pend enough money to go on safari then don't 'cheap out' on your hardware, especially if the critter on the other end has horns, teeth or claws.