the difference between the 1st and 2nd gen heads is the intake manifolds - 79 - 80 have 2 bolts at the front of the manifold, the 81 - 89 have only 1 bolt in front and one below the thermostat housing. The intakes are not interchangeable. The 85 - 89 heads also have hydraulic lash adjusters, the 84 and older are manual adjusters. All non jet valve heads are 81 up style.