Even the race prepped Darts and Barracuda's had VIN's on them. If it has any of the original sheet metal on it you may still be able to find bits of it.

This pic shows different hidden locations used on ameican cars. I don't think it applies only to Mopars. But you may try looking in some of these spots. One that the pic dosen't show but I would look anyways is the trunk latch striker bracket if you still have it there.