My son has a 45/70 barrel for his NEF single shot. I will compare the 45/70 to the 30/40 case and see if it looks like it would have a chance of working. Have you thought of going 40/40 opening the bore to what ever would make the case straight and reloading 30/40 cases? With 40 or 41 slugs would could use heavy pistol bullets and get good expansion at moderate velocities. I think you would have a better chance of feeding from the magazine also. In times past reaming 30/40 barrels to 35 was done fairly often when the original barrel was shot out. I did a sporter a couple of years ago and chose 35 because of the better probability of feeding with no modifications to the receiver or side plate. It does feed with no issues now.

There was a long discussion about big bore Krags here:
  (You need to Login
The OP seemed to decide to go the 35/40 route but I do not know that he ever finished the project. The discussion was what spurred me on my project.
There are some knowledgeable characters that frequent this site and they site perhaps they can help also.