I think what people mean by the "flattened and lengthened" is that you seem to have stretched the car a little bit? Maybe when fitting the new background.
I think this effect is made more noticeable by the rims, which in my opinion are a bit too small. If you compare it to the base you'll notice this

Try taking a look at my
Stance Tutorial, it might help when lowering the car this much to fit the rims correctly
Other than those minor errors I think it's actually a very nice chop

The rims, although a little small, do fit well, and the car sits nicely in the backdrop,
The colour change seems to have been done nicely, and the overall execution is of a high quality
Well don - keep it up
