Yes, I had mine rechambered by a smith, but it's an easy hand ream, there are several here that have done their own, see my previous post, I edited it to add John Boy's FAQs link.... just in case you read it before the edit. I use my 45-70 BCs for shooting the little 45-70, that's what they're for!!
Read the FAQs, there are several tips to eliminate the brutal felt recoil of the 45-120, specially if you use hot loads. http://www.gboreloaded.com/forums/index.php?action=search
Tim