bullet fit is very important, or you will lead. depending on what you use for lead and lube will dictate as to how fast you can drive them. bullet shape will also affect accuracy, long noses like the "money" bullets need to be harder, like 16-1, the old govt nose bullets can be cast from 30-1 or pure. With pure lead you will find that its very hard to get the mold to fill out completely, tin will help with this. with a 45-70 sizing to .459 should be good but, a slugging of the barrel should be done to find the correct size. also to get good accuracy with lead, seat your bullets out as far as you can, I know in a lever gun this can be hard to do. Felix lube is fantastic for the 45-70!! getting a mold with a gas check will help too!
The MRI lead is usually pure, if your going to shoot it with out a coating dont add linotype, its not needed to be that hard, Antimony has a tendency to lead too, if you look at the RCBS 405 FNGC or RCBS 300 FNGC (perfect for lever guns) that bullet cast at 20-1 with felix lube from my lever gun, sharps, shiloh and pedersoli all shoot 2-3in groups, (maybe 4in at worst) at 200yds and just stack them at 100yds.
matt