1968-72 are basically the same. There were some slight revisions, most notably the tail and parking lights. The grille was slightly changed but most people can't tell the difference.
1973 was a more substantial revision. The fenders, hood and trunk lid stayed the same but the front and rear ends look totally different. Larger front and rear bumpers, a different grille and new tail lights. The vent windows on the doors were replaced by one-piece door glass and the rear quarter windows was made larger and more square in shape (1968-72 was more triangular). What you don't see is a new floor pan. 1973 actually is very similar to 1975-79 because the passenger compartment and trunk floor pans are the same.
1974 is mostly the same as 1973 with some very minor differences.
Take a look at the galleries on my site to get some pics: