You certainly need a chamber cast from cerrosafe. But send it to Buffalo Arms Co. They will help you identify the cartridge, and there is a good chance they will have reloading dies in stock.

I went through this not long ago with a .40-70 Sharps bottleneck. I told the story about my quest here:

John Rigby & Co. 40-70 Bottle Neck Single Shot Rifle #15130

Good luck, and please report back.

Curl