Something else to check. Chev harmonic balancer outer rings have a habit of slipping making the timing mark position incorrect. The line on the balancer can be checked for correct position several ways. Bring #1 cylinder piston to top dead center, balancer line should be at 0 on the timing tab. My preferred method is to remove the valve cover on #1 cylinder and rotate the engine watching the rocker arms on #1, when exhaust valve is closing and intake valve is starting to open timing line on balancer should be at 0 on the timing tab.