What's it in? If it's a '68 or later and the original engine, it's probably a 302. If it's a swap then it's up for grabs. Like Terry said, check the stampings on the engine and the heads. BTW, just to rule the obvious out, it is not a Boss 302. They have Cleveland heads and, thus, a different shaped valve cover. Can't tell you how many times I've been at car shows, including Mustang shows, and had some "expert" stare at my 302 and ask if it's a Boss. I'm always tempted to say yes and spin a story about the rare '69 Boss 302 convertible, but I always smile and explain that the Boss 302s were all SportsRoofs and had a special engine.