I just read four pages of replies to your thread. I just got into the 45-70 game a few years ago when I bought a vintage 1895 Marlin in 45-70. I did a lot of research on the .45 caliber bullets and bullet performance. I'd taken the rifle black bear hunting on a couple of trips, however used the 35 Whelen instead. Last year I decided that I was going to use the 45-70. I decided on using either a 300 grain Hornady hollow point, or the Barnes 300gr TSX flat based hollow point. I liked the Barnes TSX, however was uncertain about what its performance was going to be like because of the large hollow point. Because of the uncertainty of the Barnes performance I decided to back the Barnes up with the Hornady bullet if needed. I was presented with a broad side shot at 60 yards and took the shot with the Barnes bullet. The bear hunched up, then turned and started to walk away at a very slow pace. I knew that the shot went where I wanted it to go, yet that bear was walking away like in slow motion. It went about 20 feet and then fell down, I figured the bear was down for the count. After about a minute the bear tried to get up and fell down again. I'm like stay down as I don't want to have to shoot you again!! After another minute I am figuring this hunt is over, but.... then the bear tried to get up again getting its rump off the ground and almost standing and it feel a second time. After about another minute the bear started to get up again, and at this point I tried to spine the bear with the Hornady bullet because I feared that for some reason either the shot didn't go where I "thought" it went or the Barnes hollow point bullet failed to penetrate. The Hornady bullet traveled from between the two rear quarters and came out the nose; so at this point this bear was really dead. So... this 170 pound black bear (by the way it has been reported that black bears are easy to kill) sustained a perfectly placed shot with a bullet that worked better than expected take two rounds to get the bear to stop moving. I know this is a long story, however only written to make a point that not all bullets work as they are supposed to, not all game is as "easy" as projected/reported and not all hunting shot placements go where they are intended to go due to conditions that no hunter could possibly anticipate.
I would recommend a modern day approach and use a monolithic bullet either the Hammer or the Barnes TSX bullet. I have attached a photo of the exit hole of the 170 pound black bear that I took this past season with the 300 grain Barnes TSX hollow point. And...I would love to have the opportunity to do a bison hunt and would definitely take the 45-70 and hunt with confidence with the round. And... no matter what bullet choice you make good luck with the hunt, and please bring back some photos of your hunt and share them with the group.