I am not sure what you are getting at, but usually if you download a car it comes with its onw shape, or if you download a livery the readme file will dictate a carshape that has to be used. You can't just put a livery on any car, without making several changes to the way the textures are mapped on the car model.
So check the zip/rar/whatever file to see if comes with a carshape, or if you need to use a specific carshape. Then checkout this thread for instructions on how to get it into the game:
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www.grandprix3.com]
It's only after we've lost everything, that we are free to do anything.