Kevin,
I killed two bulls last August with a 75-pound Howard Hill longbow and 860-grain arrows. The heads were Grizzlies, sharpened and honed 'til the edge was smooth as silk.
Both were heart shots, and had to go through heavy bone to get there. Neither bull made it 20 yards.
As stated above, 70 pounds would be bare minimum, and it would actually make me a bit nervous, but would probably work, in most cases.
Priority number one is arrow energy.
Priority number two is absolute perfection in shot placement. The area that will kill a bull is much smaller than the kill area on a small whitetail.
Too much angle and you glance off the ribs. If you try to get an arrow to go behind the ribs and up into the heart and lungs, the arrow will never get there (way too much body mass).
If you shoot a perfectly broadside bull one inch behind the crease, you will hit nothing but guts.
Practice shooting right in the center of the shoulder muscle. That's the sweet spot, but you still have to go through a half a foot or more of thick fibrous skin and ultra-tough muscle, and then most likely through a rib, but in that area the ribs are not 1.75 inches thick.
Keep your eye on the magazine stand, I have an adventure-filled Australian buffalo story coming out soon in Bowhunter Magazine that should get your juices flowing.
P.S. Man, are you going to LOVE buffalo hunting!