Welcome aboard Dangerdan.
Did you bell your case mouths enough before seating the bullet? Whose dies are you using? The major brands have a belling die set up for jacketed bullets. You should be able to seat your projectiles without buckling the case if you have a slight bell at the lip of your case.
To be honest I've never seen that happen with a 45/70 case, but it is common enough with thin walled cases like the 32/20 and 44/40.
Why not just toss the round. In the grand scheme of things, that case will not be a huge loss.