Toolmaker
You might troll the posts at the ASSRA site for .40-70 SS articles.
When I bored the Hepburn the only possible brass was the 9.3X74R brass. That brass has a thin rim so it was a little sloppy in the Remington.
Today we have Hornady .405 brass when you can find it. It has a fine heavy duty rim. I consider it better than the metric brass. It does require a deeper rim recess than original .40-70 SS brass.
Boring the chamber was not very difficult other than the potential of destroying an unreplaceable original barrel.