Was the head surface ever checked when the engine was apart and was the head checked for cracks? I am not sure that what the torque specs are for your engine, but 58#'s is fairly good. I'm not saying it is enough, but it is adequate, especially if you have not run it hard. I'd retorque and if that does not stop the leak, I would take the head off, check to make sure it is not warped (can do this with a good steel straight edge, and then have it checked for cracks. A good machine shop should be able to do this for you. Just make sure they are familiar with aluminum heads. Good luck. Oh, when you retorque, you do need to back off each bolt and then retorque as you mentioned.