I started out in BPCR with a 40-70SS. I like the cartridge a lot. Stick with the modern bore diameters such as .400-408" as there are more bullets and reloading equipment choices available.
I think it's easier today to get the 40-70SS shooting well by using the Hornady .405 brass trimmed back to 2.5" although I don't know that from experience. I started with stretched Krag brass and neck thickness was not consistant. If you're going to have the work done in Alaska, I would order a reamer designed to use the Hornady brass.
The 40-50 bottleneck is cute cartridge but limited for use. My shooting pard has one but cases are a pain to make and have to be neck turned as well.
As far as barrels go, like Brent said, you can't go wrong with any of them. I don't think that GM will do a custom profile like Douglas or Krieger. It took 4 mos. for me to get a custom .45 cal Krieger barrel with 17 twist.
