Direct vent fireplace into brick wood burning chimney?

Hi and thank you for your help.

I am trying to install a " napoleon " direct vent fireplace into an existing wood burning chimney. The problem is that the insert is bigger than the actual opening, it does not fit in. I was thinking to remove few bricks from the side and the above opening inside wall.
Is it safe? Does it cause any damage to the outside walls and the general chimney structure?