66 Cuda VIN location

340mopar said
There has to be a place for some sort of hidden VIN number, a riveted tag would not be enough to satisfy the feds.

NOPE, not until 1969. As I said before (and I have a car with enough documentation to back this up - my 68 GTS convertible) the "hidden" numbers on a 68-older Mopar are the SHIPPING ORDER NUMBER, not the VIN. You need a broadcast sheet to match the VIN and SO.