Buffalo live low in their chest - similar to goats. The tendency is to shoot them too high. If you do a google you might come up with a cross section, showing organ placement and bone structure.
Personally, I'd prefer a rifle, but a smoothbore will work, of course. 65 to 75 yards is well within the accuracy range of a well-loaded smoothbore. I'd be using 110 to 130gr. 2F with a hardened ball - ie; straight WW alloy will work fine. It doesn't have to be harder than that. You want the ball to traverse the lung cavity with enough speed to damage tissue from the shock wave in front of the ball, not merley punch a 1/2" hole.
Buffalo can be tough, but if you put a large diameter ball or bullet through one's 'lights', they usually go down quite quickly. Friend of ours, 6'6" tall Sam C., killed one with a .62 Hawken Taylor built him. One shot was all it took and the buff dropped quickly after the shot. Sam used something like 150gr. 2F on a 50 yard shot. His Sharps worked allmost as well on another buffalo.