The difference, by the numbers
They have no ingredient in common: the Harvey Wallbanger has vodka, orange juice and Galliano, the Seven and Seven American blended whiskey and lemon-lime soda. In parts, a Harvey Wallbanger is 1 vodka, 4 orange and ½ Galliano; a Seven and Seven is 1 whiskey and 3½ lemon-lime soda. By volume, the Harvey Wallbanger is 73% mixer and juice, 18% spirit and 9% liqueur; the Seven and Seven is 77% mixer and juice and 23% spirit. Different bottles, similar balance: that makes them close cousins on the ratio map.
The Harvey Wallbanger is the stronger drink in the glass (10.1% ABV against 9.0%), but being smaller it holds about the same alcohol (1.03 and 1.01 standard drinks). The Harvey Wallbanger has more calories (about 176 against 163), but the Seven and Seven has more sugar (16.2 g against 15.1 g). Both are built and served in a highball.