To remove the valve covers, you want to be sure to take off the rocker arm fulcrum bolts and rocker arms. If only the pushrods need to be changed, then first and foremost, the fulcrum bolts would be loosened then the rocker arms turned sideways so as to expose the pushrods; they must then be correctly aligned and placed most likely in a perforated cardboard box if the car is to be used again in the near future. After that, check the lengths, wear, cracks, and other unidentified defects of the rocker arms especially at their contact surfaces with the pushrods and valve stems, plus the hole at the pushrod extremity. Inspect the fulcrum contact area for signs of wear and galling; in any case where rocker arms are worn or damaged are present, replace them along with the fulcrum seats. Using a flat surface, roll the pushrods across; check the pushrods for cracks and excessive wears and deformation is indicated by bends. When, for instance, doing an installation, grease the top of the valve stem and the pushrod guide with engine oil or assembly lube and the fulcrum seat and rocker arm socket. The pushrods should be fitted next and then the rocker arms; also the fulcrums and screws for the fulcrums. Turn the crankshaft until the lifter is completely down, and then tighten the fulcrum bolt to the initial torque in two passes. Last, reassemble both valve covers and gaskets with newer ones, start the engine of the car and check for roughness or other kinds of noise.