Even though you don't see anything visible and this thread doesn't really mention what the issues are, I'm assuming you answered your own question w/ the above. That bolt needs to be replaced....The rear exhaust bolt is broken off, has been since I got the car 7 years ago, no indication that it has been leaking, no black carbon tracks, no ticking hot or cold.
As for plugs, I run autolite iridiums. Switched to iridium after i saw a couple of platinum tips from delco double plats completely gone upon plug swap. Don't want that stuff banging around in the cylinder and the gap is WAY off once the tip becomes dislodged.