On the 68 model cars... you will need to look on the back side of the engine, near where the transmission bolts up to. Behind the intake, on the passenger side... there is a square indention that will have the last sequence of the serial number stamped there. In late 68 and early 69.... mopar began stamping this number on the side of the block near where the oil pan mounts.
Hope that helps... if you need... email me and I will shot you a picture of where to locate this number.
Tony
crittergtr@aol.com