The 68 D code blocks (from a 1968) car partial VIN is stamped on the top rearmost flange of the motor where it mates up with the trans/bell housing just behind the oil sending unit location. Sometimes it is hard to read because it's a rounded cast surface and the stamping was not easy to make in that area. The numbers on the bottom of the block are date code and assembly type info. A 426 block from a 1969 car would have the partial VIN stamped on the same area as a 1969 440 block. It's possible that s 69 motor could have a 68 casting date but a 426 motor out of a 1968 car will have the stamping on the top rear flange of the block.