A project that started nearly six decades ago and gave birth to a series of fascinating mid-engine sports cars, the Mach 2 remains one of the biggest “what-ifs” in American automotive history.

I realized I was falling in love about three miles in—that’s when it really hit me that this could be the one. The one I’d pick day-in and day-out, the one I’d share miles and tears and laughs with. I ...