according to my hollander, the 40 models frame is :#344. the frame number for the 41's is 345, except for the 4dr sedan, which is 346. this is the same frame number as the 46-48 models.
the 344 and 345 frames interchange.
i believe you would have to have the frames side by side to see what the difference is between the 344,345, and the 346. i don't think there could be much. however, i don't know what the wheelbase differences may be.
hope this can get you started in some direction.