That frame is tough steel. I'd tap the hole before the screw. My experience has been( the usu headaches w/two showroom stock camaros, now the Zo6 w/ transcooler install) the self tapping are usu made for sheet metal, not for the thicker tougher frame rails. IMHO.
Drive the left or right side of the car up on some boards. Put the rocker rail in place and use your jack to lift the car with the rocker rail as the support , this will keep the rail in place. Now drill your holes and use loc-tite on the screws and secure it to the car. Do the same for the other side of the car or you will have alot of extra holes. Make sure you use a drill and some sharp drill bits to start the holes or you will never get a screw into the frame. Good Luck! :cheers: