I would use maker expanding copper subs 400 or 500g in the 45-70. The bullets are so long that a shorter case should have no real effect. Could probably just cut down brass and use in a regular 45-70 chamber. Note Hornady already does this a little for the ftx bullets. Brass is shorter so they can keep the 2.55 oal.Sounds like what I'm after, is there a name for this cartridge?
I have an an encore and enjoy it so it would be ok to get a new barrel for it. A friend has a 357 max barrel for encore and has used it successfully for whitetail
I have a couple .475 Linebaugh, which is a 45-70 case shortened to 1.4" Due to the taper of the 45-70 case the inside dimension at that point fits a .475 bullet.
For encore I would look at the 360 buckhammer. Although due to twist you are limited to just over 200g bullets.