Can you find a .40 cal taper crimp die?  I shoot a couple of rifles with chambers for .45 cal bore diameter PP bullets.  Regular sizing dies don't quite take the case mouth down enough to chamber.  I squeeze the mouths down a little more with a taper crimp die and then fireform the brass.

Chris.

I'm coming to this thread with much less experience than others but I have a 40-63 that I understand was the same outside dimensions as the 40-70 Ballard.   I used Hornady 405 cases, cut them off to lenght, trimmed and sized with C&H4D 40-63 dies so they'd chamber for fireforming.  Then, per several posts I found here, I used an inside neck reamer to thin my case walls to accept a .410 cast bullet.  I leave some as-is for paper patch bullets.    Shoots well and my reamed case necks are still about .008 thick, as measured with calipers (not so accurate as other methods of measuring) but plenty stout enough for repeated use.