Some of the 4.3L engines are going to require self-aligning rocker tips similar to those required by the Vortec engines. The self-aligning rockers will have two thin washers on either side of the roller to keep it from sliding off the top of the valve. Your stock rockers will have a slight indentation in the tip to keep it on the valve stem if it is a self aligning rocker arm. On the older, non-self-aligning type engines, the holes the pushrods come through in the head are sort of rectangular and hold the pushrod straight up and down which maintains the alignment of the rocker.