That's not entirely true. Lightning McQueen and Tow Mater have mouths where there grills would be. Most cars in the movie are like that. Or the grill is incorporated into there design such as Doc Hudson or the Sheriff who both have mustaches for grills.
I think you could even try remaking it and possibly add the ECTO-2 as ether the same car with a makeover or a new car.
There is also a little problem with the concept. Where is the Proton Pack? What if you made it that the Car's roof stuff where a cross of the original and the Proton Pack that it could fire from the roof?
But for a start its a great concept and it gets the , no pun intended, wheels in motion. You could take allot of famous cars and do this, even planes and boats and truly add to the world.