Welcome aboard Matt. While Adam already answered that there are 3 identical body styles, fastback-notch-`vert, they each have unique features for each model year.
67: No side marker lights Grill is a square grid Thin cast metal trim centered between the tail lights Small reverse lights on either side of license plate Plastic window moldings press into window rubber Grooved hood inserts
68: Round side marker lights Grill has vertical ribs Trunk trim is an upper & lower aluminum piece Reverse lights are part of the taillights Stainless steel trim for window moldings Hood inserts are allen socket heads with ribbed or engine size 340 or 383 centers
69: Rectangular side marker lights Grill returns to square grid Shape of grill changes from (0=X=0) to (0=Y=0) Trunk trim is wide aluminum panel from side to side Larger reverse lights return to either side of license Flat hood without inserts Center of the hood peak has a pronounced hump
This should get you started in identifying the different model years. I didn't get into details such as idler arms, fuel tanks, or engine sizes per model year for the sake of simplicity. If you need more refinded details, ask away.
There's a lot of knowledge on this board:thumrigh: