For every person you ask you will get a different best load.
I don't load any of my 45/70 ammo to high speeds. I like lead bullets in the 405 and 475 grain areas. Run them at modest velocities. I also load some remington 405 JSP to around 1600fps for when I think I need them. Best thing I can tell you is work up a load that is accurate at your intended distances and concentrate on shot placement. The most important part of all. All of my 45/70's are single shot except my BFR revolver. My bullet choices are more varied. There has been more than one gun writer who likes the Lyman 325 gr HP for hunting. I have the mold for them and they do a good job and are easy on the shooter. Good luck finding what you need and good hunting.