You can read all about the Mustang Boss in Mustang Boss 302, from yours truly, Motorbooks. Here's an excerpt from the groovy 70s.

The Mustang Boss saw some slight changes in 1970, which included graphic detailing. The front end became more European looking, reverting back to a pair of headlights in place of 1969's four-headlight system and adding a pair of gill-like scoops in the front fender extensions. At the back, the rear panel lost its concave shape and the taillights were recessed into a new flat rear panel. On all 1970 Mustang SportsRoof models, Shinoda succeeded in eliminating the "bric-abrac" quarter panel scoops and roof pillar emblems that had "cluttered" the 1969 version.