Hornady 350 gr. RNFP. Have not used on bears but did take 2 bison and one large pig with that bullet. Both bison were one shot kills and the the pig was hit in the front chest and bullet found in front of left rear ham, distance of 33 inches, expanded to almost 70 caliber. One bison was a complete pass though and other bullet ended up going though rib bone heart and lungs and stopped on far shoulder joint after busting the shoulder. I was loading the 45/70 to a velocity of 1850 fps out of my Marlin 1895 LTD-5 lever action.
Skip the above information, looks like Midway and Brownells no longer carry that Hornady ammo or bullets in the 350 gr. weight. You may have to settle on a commercial 405 gr. load. Last time I bought the 350 gr. RNFP bullets was over 7 years ago. Hornady is now only listing their 325 gr. and I have no experience with that bullet.
If you go on the Marlin forums, there's a lot of info regarding bear loads. But the general consensus is 405 grain flat nose like Remington makes. I have a Remlin 1895 gbl and find the Hornady 325 grain ftx to be extremely accurate. Sub MOA at 50 yards, so that's what I carry in black bear country. I also have the HSM Bear Loads in 430 grain. They have substantially more recoil and wallop than the Hornady's, and undoubtedly would do the trick with a well placed shot on a black bear.