68 was the first year that the vin code had to be stamped into the motor and transmission .On the back of the motor there is a flat machined spot that should have the vin stamped into it .It will be hard to see but you may be able to read it with a mirror ,it will be the same number as stamped on the fender apron. An automatic transmission will have the vin stamped on the top edge of the tail housing where it bolts to the main body of the transmission .It will be on the top so it may be hard to see it .4 speeds will have the vin on the mounting flange where it bolts to the bell housing ,usually on the top of the flange