I hate to get back into this again, but . . .
Let me be as clear as I can: Your .45-70 is ILLEGAL for buffalo in Zimbabwe under current law! See my citation above of the Zimbabwe Firearms Act. All three of the web sites you linked to are the outfitter's info page, not the game laws. They all mention minimum .375 for dangerous game, and I can assure you that the people who wrote those pages meant the .375 H&H. If you show up on a buffalo hunt with a .375Win (for example) your outfitter will throw a fit or question your sanity, or possibly both.
The game laws of Zimbabwe, Tanzania, and most of Southern Africa specifically require BOTH a minimum caliber AND a minimum energy. Neither a .300 RUM (which has enough energy) nor your .45-70 (which has enough diameter) are legal for DG in Zimbabwe or Tanzania. Your DG cartridge needs to have BOTH THE MINIMUM ENERGY AND THE MINIMUM DIAMETER!
A .45-70 shooting Garrett Hammerheads, if it achieves full published velocity, will be 16.9% short of the required energy for Tanzania and 18.1% short of legal for Zimbabwe. This is about the same energy difference that you will find between the .22-250 and the .243. Both are capable of taking deer under most circumstances, but the .243 is legal almost everywhere, while the .22-250 is not. The good news is your .50 Alaskan seems to fit the bill legally with several loads, so your next Cape Buffalo hunt will likely be legal.
Frankly, this isn't even my fight. I think the .45-70 ought to be legal for DG. Hell, I think hunters ought to be able to hunt DG with anything they feel they like, sharpened sticks and baseball bats included. It's no skin off my nose if hunters come home in pieces, if they do so after taking a rational look at the dangers and deciding to roll the dice. You seem to be above the age of consent and I have no interest in standing in the way of an adult's entertainment, even if I think that he's just looking for a creative and interesting way to assume room temperature.
I am, though, surprised to see "the original bastion of 'Enuff Gun'" playing silly-buggers with the bottom end of the minimums on something that can kill him, and I think it's irresponsible to recommend the same game to a novice that's looking for info on getting into the sport.
Just my two cents worth.
Draw quickly, shoot carefully.
If you can get closer, get closer. If you can get steadier, get steadier.