Since the models are covered, I'll cover the real cars. The major difference is the side moldings. The '68 had a molding that flowed off the corner of the grille, and continued down the side of the car. Majority of them with this molding had black painted rockers. '69 Got rid of the side moldings. '68 Had side marker lights with the engine callout, the '69 was just the lense, and the '68 had a mesh grille whereas the '69 had the split grille similar to the '70.
'68:
'69: