Fireplaces, Heating Stoves, Flues and Chimneys - Insert is tight fit, can I cut the damper throat?

I am installing an Osburn insert in my fireplace. It is slightly on the big side so the stove won't slide in all the way. This means the outlet pipe overlaps the damper throat plate (the fixed part, I already removed the moving part) by a few inches (if you were looking from the top). The vertical distance between the outlet pipe and the damper is short enough that it is very tough to bend a SS pipe that sharply. I want to cut back the damper plate and some of the brick above to ease the fit. I'm worried that if I cut through the damper it will structurally weaken the chimney. I thought fireplaces had a lintel that supports the weight separately but I haven't been able to confirm that. My fireplace has an obvious lintel that holds up the rock front but its not clear what supports the brick chimney across the opening other than the damper. I'd really appreciate any input and insight anyone can offer on this situation.

When dealing with the structural elements of a chimney, a sweep is the best person to recommend any action.