Check my post on page #2 - 29/03/05, it will give you a lot of good suggestions and thoughts on the subject.
I have a #1 in 45/70 and wanted to rechamber it to .450NE 3 1/4 but decided to rebarrel as the Ruger was a little light (6 1/2 lbs.) and had a 45/70 twist.
I bought an octagon blank, rented the reamer, threaded and chambered it and I am in the process of hand finishing the flats right now.
Still have to make a forearm, mount sights and rust blue it so it will be a while before I can actually shoot the gun but I'll post the results when I do. Dave