Early Broncos (built in '65) 1966 models will usually have the Vin tag on the driver side kick panel., just to the left of the emergency brake pedal.
Later on it was moved to the glovebox door (big mistake) and also stamped on the top of frame, by the starter.
The eng. # (if orig. eng.) can get you close. i.e. C6------- would be a '66 eng.