Interesting discussion.
I, for one, have never heard Southwest mention anything other than the 73 in any of their material since they flew 727s for a short time way back in the days of the Dodo. I also understood the 767 production line to be closed (except for freighters), so they would have to scrounge up used ones (not that that would be too hard). Flight frequency has also historically been a part of their selling point, so replacing several 737s with a 767 would seem to conflict with that, but who knows?