Anyone know if on the 1966 El Camino there is a Vin number anywhere on the frame? I know that there's nameplates on the firewall and the door jab. But, are there any other locations?

The plate on the firewall is not the VIN but rather the Fisher Body number plate. It has nothing to do with the VIN, only the body preparation (paint, seat type and color, etc.) There are places on the frame that aren't readily visible but there are some 66 gurus here that might know exactly where they are.