Unless your brass is separatin and leaving the body in the chamber, you probably have an extractor issue, not a chamber issue.
Chances are, your fix is possible with the presant chambering. Your gonna have to do sme footwork, but its better to find a good smith than to wreck your gun for good by boring it out.
If you really want a 45-70, get a new one, refrain from hackin up your classic.