Need advice on wood insert intallation with factory chimney!

I have a factory chimney with a typical wood burning fireplace.
I bought a Napolean 1101 wood insert and homesaver ultra pro liner.
The problem is that factory chimney has a sheet metal "canopy" that kinda forms a funnel up to the flue... but the front to back depth of the canopy is about 1 or 2 inches too narrow to allow the wood insert out-take (on top) to have a clear shot up wards...
If I modify the sheet metal canopy by cutting some of it away, will that be safe? .... the insulated liner will carry all the hot gases away, and the insert it self is double walled, so do you really need the integrity of the canopy surrounding the insert at the bottom?
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