Check the VIN. It should read 137 for a Chevelle Malibu Sport, then either 37 or 57 for the 2 door hardtop, then the last number of the model year, ie 8 for 1968 or 9 for 1969, the next number is the assembly plant, and the last 8-13 digits are the sequential build numbers.
So the VIN should read something like this: 137578T007635
You can also check the cowl tag. The year will be right next to the letters "ST" located on the left. It will say either 68-xxxxx for 1968 or 69 for 1969.