I'm on the edge of my seat reading through this one. I have an old belgian hammer double that I have chambered sleeves that I will fit, some day. I have them sitting there looking at me and laughing that I don't have time or the talent and tools as 450_360. Mine are also .45-70. I am looking forward to the regulating as well.
Looks like a great project an should make a wonderful classic double. best to your effort.