1. that picture is waaaaaaaay too big
2. it almost seems like you used the high-res backdrop and then pasted your chop onto it and blew up the car, which made the car very very blurry
3. the seams between the patches of car you stretched and moved around aren't at all difficult to see, so try going back in PS and making all the body lines match up smoothly, it'll do wonders for the way the car looks
4. i LOVE the shape of that cabin, it's very sinister looking, but as mentioned in #3, MAKE THE LINES MATCH UP!
good ideas, just needs some tinkering on
if you're not completely finished with this one i hope to see a revised, scaled down, version in the near future