The simple fact is this is good for internet discussion and not much else. Pretty much every country in Africa has a caliber and energy requirement and it is usually right around a 375 H&H magnum.

Bell was a great hunter and I will never discount what he accomplished; however there are plenty of other well respected hunters that believe(d) bigger is/was better.

Shoot premium bullets.
Use the largest caliber you can shoot proficiently. If you cannot handle a properly fitted 375 maybe you should stick to deer hunting.
Use a cartridge that will drive the premium bullet to sufficient velocities.

One other note, there is a such thing as not big enough. Over the years I have killed around 2000 alligators, most with a 22 WMR. That is about the same number of elephants Bell killed, so I have a good sample size. I just finished my season and shot 66 with a 22 magnum, as usual no issues. The previous two years WMR ammunition was difficult to find so I used 22 long rifle. On a big alligator a 22 LR is just not enough to consistently kill one.