Chassis VIN is stamped into the frame before the frame is painted. While some of the letters/numbers may have gotten partially filled in by paint, most of the VIN should visible once you get past the grime and dirt that collected on the frame over the years.
The magenta arrow in the following picture shows where I usually find the chassis VIN. The chassis in the picture is a 1969 M300. I have found the chassis VIN in the same place on a 1972 M375 chassis and a 1973 RM350 chassis.
The following are close-ups of the chassis VIN on the 1969 M300, after the frame has been cleaned and stripped but before being prepped for painting.
Note: Mopar changed the format of their VINs between 1969 and 1970, so this is an example of a pre-1970 VIN.
