Boot Rail in the way

I'm attempting to install a new bathroom exhaust fan. When I went to cut out the new hole, I noticed a strip of metal in the way. After some research, I believe the metal is one of the register boot rails. What are my options at this point? Its a small bathroom so I didn't want to have to relocate the duct boot. Could I just cut out that side of the metal rail? Or is there a better quick fix for the issue that I'm not aware of? I'm against a joist for the bathroom vent so I cant expand on a different side.