Yeah.. the 5.3 is a different situation as is the 4T65 in it. Pulling the trans is much tighter and therefore not as much fun as a 3.8L. It's possible, just more of a pain. As for the leak... (experience talking here) it could be the seal or the converter. Yes.. I've done a seal and ended up going back in for the converter only to put pressure (on it and see it leaking from a crack that was not visible to the eye.
As for the o-ring, I'm trying to remember.... pretty sure that would leak back into the trans itself. So it wouldn't leak externally due to the ring.
