I found this information on the National Traffic and Motor Vehicle Safety Act of 1966 (effective January 1, 1968) concerning the engine VIN.
Public VIN (since January 1, 1968 visible through the windshield)
In the United States, the National Traffic and Motor Vehicle Safety Act of 1966 (effective January 1, 1968) mandated certain safety requirements on vehicles to be sold in the USA such as side marker lights, safety belts and Vehicle Identification Numbers (VIN) visible on the dash through the windshield. This is VIN location has come to be known as the “public VIN” and is probably the VIN that you are most familiar with.
Hidden or Confidential VIN
VINs have been stamped into frames of vehicles for many years, however the process became more uniform starting approximately during the 1968 model year. The VIN was stamped into various metal objects on the vehicle, including the frame, the body, the engine, transmission and other places. The VIN on the frame or the body became known as the Hidden VIN, the Confidential VIN or the Federal VIN. This number is usually not a full, complete VIN but a derivative thereof. The sequential production number of the hidden VIN should match the sequential production number (the last five or six digits) of the Public VIN
I also found some internet posts suggesting Ford began stamping all blocks Jan. 1st as well. While not 100% definitive, I would say that if the VIN isn't stamped into the block, I don't think you can claim yours is the original block, as you have no way of proving it.
Steve