 There are two variants, the Boss 302R (Racing) for use in the IMSA Continental Tire Sports Car Challenge and the Boss 302S (Sprint) for other racing series such as the SCCA Pirelli World Challenge GTS class.Introduced in November 1970, the Boss 351 Mustang became one of the last high performance models of the Mustang line.It was based on the Mustang SportsRoof model and replaced both the Boss 302 and the Boss 429 models.