Very nice step by step pictures. Thank you.
Cool project and great results

As to "what will the Red Label take" I have seen 20ga in 7x65R, 300, 338win on up to 500/416, the 12ga in 500NE, there are way more than a few 45/70's out there, besides the Heym, Paul Jaeger or Ruger built.
Butch Searcy built some as did Rogue River I believe.