The last five digits of the original Ford engine number match the last five digits of the VIN/chassis number, certainly on normal cars - don't know enough about things like Cosworths where the engine comes from an outside supplier.

The VIN/Chassis number is the one on both the VIN plate and stamped in the floor - these can't be changed. The engine can (obviously be changed, so doesn't appear on the VIN plate.

If the VIN plate doesn't match the number stamped in the floor you have a problem.