I had a notable fella that frequents this froum, run the numbers on the loads that "theoretically" my 444 Marlin with a 16 1/2" barrel would be able to produce after modification . He has all the computer software programs and expertise to give some very interesting data on ballistics. At the low end, one of the loads was a 325 grain bullet at 2050 fps. According to all his calculations, that bullet hitting a big bear at 150 yards, would produce the ballistics of a 30-06 at point blank! I was amazed! In his opinion, the modified Marlin would be a good bear gun out to 150 yards....providing of course that the shot was well placed. My goal was to have a good maximum 100 yard bear load for the Marlin, so being conservative, this load would do the trick..........Now, just look at the 45-70 or the 450 Marlin in a levergun and compare the ballistics against my load. I would see no problem what-so-ever with the 45-70 or the 450!.........For some great info on the 45-70 go to Randy Garretts site: garrettcartridges.com, and then go to Lupo Safari's...(at the bottom of the box on the left side of your screen).......If you read of his expereinces, you will see that the 45-70 or the 450 which is a ballistic equivelant of the 45-70 can pack enough punch to take anything that walks the earth!!!!
I just went back through some of my research data. It seems that any bullet of 400 grns or heavier at 2150 fps is the "standard" for taking all "large" African game. I have seen some 350 grain loads for the 45-70 that run around 2100 fps, and some 405 grain loads that come darn near close to that, and I would surmise that for the big bears that would do the trick.