RE: 45-70 in Africa.
Buckshot,
Read upstream and you'll find the answer to your question, "killing power" does not live on penetration alone, especially when the game may decide to dance on you before it dies. The prohibition on solids by many states for big game hunting is attibutable to the problem of wounding/killing of multiple animals (even with modest cartridges) and the fact they do not transfer their energy particularly well.
I really would encourage the two of you to go over together and get out of the fluff and into the bush and give it a go and keep track of each others comfort levels as you approach the black.
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AK,
You were doing pretty good there for the first 80%....
Good Luck and Good Hunting,
EKM