The side shot in the first post shows the edge of the headlamp bezel - only the square light bezels are that thick, the round light bezels are only about 1/2" thick at the side.
I converted my old '80 K10 from round to square, very distinctive look. I've always liked the square lights better. IMO, they fit the trucks better because it's shaped like a brick, there's nothing round about the 73-87(91) trucks. You've never heard of anyone refer to this series of truck as a "round body".