[70GMCBB]

Make sure your negative battery cable ties directly to the engine block. I've seen it connected to an accessory bracket and sometimes directly to a head. If I remember correctly there is a ground strap on the back of each head connected to the frame. I've also seen a smaller wire from the negative battery cable at the battery going to the frame. This is a start anyway. I'm not sure but there might be ground straps at the back tail lights also.