To compositie it you would have to try to match the lighting form the picture your using, look at where there are light spots/highlights/shadows on the clouds, is it direct sunlight or diffusing through clouds, all will make a difference.
Its a bit hard with just a sky background but you can get an overall idea of the lighting colours/direction etc.
What you could do is make an environment for the cockpit/spitfire to refelect using the background of the pic as the environment/ or add it to a plane to reflect.
You could use the backgorund as an image plane with fast renders on a low setting to get the overall lighting set up before cranking up the render settings and rendering in layers to give you much greater control.
these might help to explain it a bit better
"No pressure, no diamonds" Thomas Carlyle