If they are matte paintings and you need a very long one, one that's too big to create on a single layer, then you just stack them up in layers so the edges match and animate the position and in and out points so they match.
Another option is to use something like Trapcode Horizon to create a seamless background.
A third option is to match the camera angle, position, and movement with a real camera and then shoot a background plate. That's the way they shot in studio car scenes since they started doing it.
As far as setting up the matte, just render your stuff 3D with an alpha channel and set up the proper layer structure.
...and don't forget to match the lighting in the 3D application to the environment in which it will be placed in AE. Sunny Day... Cloudy Day... Dusk... Night... all different lighting looks on a shiny surface like a car.
Thank you Mr Gerard for your solutions - you suggested quite a few options, which are appreciated.
Thank you also Mr LaRonde.
This forum is great!