Is there anything that I have done wrong or it's just the design of the parts I have used?
Well the Falcon parts don't really fit. They're kinda off perspective I think. And you can plainly see they're from a studio shot. They have no reflections or anything to suggest they are outside on the car. It just looks stuck on there.
i quite like it, yeh the front is out of perspective and needs to be slightly darker, i think the two cars fit together well, if a little bit more time was spent on it, it could turn out pretty good, nice idea