Jeff, the book is no doubt referring to pre-68 trans, which use a "coarse spline" yoke, as opposed to the later trans [like your 68] that use a "fine spline" yoke [title says 55-60 trucks] .... if the MY6 is pre-68 yu'll need the coarse spline, post-67 you'd need the fine spline, check a wrecking yard, maybe even a FLAPS
Bill
