I have used 400 gr handloaded 45-70's with a Marlin lever action on big black bear and walked away wanting more gun.
The soft nose bullet loafing around at 1800+ fps was concerning regarding penetration of shoulder bone and shouldermuscles requiring several shots.
Personally I would want to go with a Ruger #1 at 2000 fps with a 400 gr 45-70 premium bullet or similar set up in a bolt gun as a minimum before considering Africa with a 45-70.
And that kind of velocity for a 45-70 is loaded HOT!!

Just my 2cents----