If I may make a suggestion...rechamber to 45-120 (45 3 1/4") and that will have the 45-70 rim size, use 45-70 shell holder, use 45-70 dies to "neck size" and still be able to be loaded to about .450 ballistics. Also brass is readily available from Huntingtons and a few others as the .45 Basic. Just buy a trim die and run the cases through them and you are ready to shoot. I did this several years ago with a Ruger No. 1 in 45-70 and the only problem was that at around 6 1/2 pounds and full loads it damn near brought tears to my eyes each time I touched it off....