Big-block clutch forks require length and more meat. Shown here are two basic big-block clutch forks. Left is for a 390 High Performance. On the right is a 428 Cobra Jet. The 390 fork has pivot holes designed for '67 (before the Cobra Jet). The Cobra Jet fork doesn't have pivot wire holes. These forks are interchangeable with minor mods. They have identical stampings.