I would say the 6.4 at 100 yards is good but then I am very new to .45-70.
Mine is a trapdoor with the original barrel and I think I won't get that group until I do a little more fiddling.
I mold my own bullets too from a Lee 405.
I do size as I said and I am not certain the leading comes from too small a bullet but everything else I do is the same as for my BPC revolvers and for the Winchester clone (.45LC) I know the bullet is undersized for the lever gun and so I went to a larger sizing die for that. I have not had a chance to get into the shop and make some bullets for the lever gun and the Trapdoor yet, but I am hoping by beefing up my lube and sizing the bullet a little larger I should get the speed up and get the leading under control.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson