It would take a lot of work to make it fit. If you are doing this to get a particular road name/paint version for a Lionel chassis, you would be better off just getting an old Lionel shell and repainting.
The K-Line/Kusan shells came all the way down to cover the frame/chassis. Lionel's FA1s were designed to have the frame be part of the side of the body. So, the sides of the body are noticeably shorter. After modifying the K-Line shell at the front and for the rear screw-hole, the body would sit too high on the frame/chassis unless you cut it down. Too much work as I see it.