I have not hunted in Africa, but I doubt that longer barrels will offer much in the 45-70. It is not fast in the heavy bullets even full of powder, so velocity loss in shorter tubes should be negligible. I was surprised by the low recoil of my guide gun when I first fired it, but Buffalo Bore and Garrett cartridges will liven that up a bunch.

Good luck with the Cape Buffalo. I hear they are very dangerous. Take a buddy!