Leading is a tricky subject, and unfortunately there's not a universal answer. Leading is a function of bullet fit, bullet hardness, bullet type, and velocity. Too slow can produce leading just as easily as too fast. The reason for this is if not enough pressure is generated to expand the base of the bullet and seal the bore(obturation), gasses will get past the base and cause leading. I experienced this with my .357. Conversely, too hot of a charge will melt the base and cause leading. If a bullet is too sloppy in the bore it will leak. If a bullet is too hard for the velocity you're trying to shoot it won't obturate properly, causing leading. If a bullet is too soft it won't hold the pressure properly and will lead.
So to make a long story short, you'll have to play with your charges and tailor the for YOUR gun. Hopefully some 45-70 shooters will offer more advice than I can give.
As far as what type of bullet, I think a cast SWC will do the deed for you.
GH1