Brad:

While it would be nice to have the rifle chambered in a Brit cartridge, the least expensive alternative is to shoot it as the .45-70 it currently is. If I read your question correctly you are looking at advice on which .45- 70 based case would be best.

The .45-70 is a grand chambering and more than adequate out to 600 yards. I shoot it as the chambering of choice for black powder target games. Beyond that the .45-90 takes over and is THE 1000 Creedmoor cartridge. Any of the longer cases tend to be problematic with black powder and unnecessarily large for smokeless. If you intend to shoot smokeless there is no need for a case larger than the .45-70, as it will hold more smokeless powder than the action will.

Glenn