Hole-in-the wall gas fire?

We have purchased a hole-in-the-wall gas fire and the instructions say that it should be 450mm from the floor. However, the chimney narrows into the flue at 1250mm. That means that from the top of the engine to the flue there is only 280mm but the gather hood is 340mm. Any advice as how to overcome this problem would be appreciated. We do NOT want to create a hearth as the room is so narrow.