Gary,
Under no circumstances should you alter the chambers on your rifle.
Early .450BPE chambers had some variation. I learned this in much the same way as you. I have a nice G. Gibbs .450NE I had loaded for many years (RCBS dies). I acquired my first A. Henry .450BPE thinking I was already set up to load for it. I ran into the same problem as you. Fired cases re-sized in my RCBS .450NE full length sizing die wouldn't quite chamber.
I discussed the problem with Stan, in the RCBS custom die shop, with whom I have had various dealings over the years. He confirmed that prior to full standardization there were several different concepts for .450 3 1/4" cartridge dimensions. I sent him Cerrosafe casts of my Henry chambers, and before long I received a perfect custom sizing die.
I have run into the same problem with the .450/.400 3 1/4" also.
Call me if you want to discuss. Proceed with great caution. These are treasures not to be harmed by our handling!
Curl
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