Do not abandon hope, ye who enter here!
Joe at Island Modelworks might do a carbody for you, if you can get up the numbers to make such a venture worthwhile. Joe makes a number of rapid transit bodies to fit the frames of others' cars of the same length; you might even be able to find something here on his website that could be adapted to your needs: http://islandmodelworks.com/Oscale.html
If you should wind up doing some "kitbashing" to achieve your heart's desire, Quentin at Q-Car has lots and lots of parts and is a fantastic resource: http://www.qcarcompany.com/
Best of luck!