Gibson guitars from the Norlin times were a bit variable, but the worst period was around 67 - 70. I usually like the sound of a square shouldered Gibson dreadnaught better than the round shouldered model. The exception for me is the Hummingbird - I've never seen one I like, they all seem too quiet and lifeless to me. The notable thing about the dreadnaughts made from the 70s on is that they had a symmetrical X brace, which means that if you find one that sounds good you can almost guarantee it will sound as good if converted for a Lefty.