727 Overdrive
He said 518, not 581. And it's the overdrive version of a 727 that your mis-naming as a 727 with overdrive. When they made it into a od trans. They no longer call it a 727. They call it a 518. Some manuals also call it a 46rh (early yrs) or 46re (later yrs).
Do a search for 518 installation or overdrive installation and you'll find numerous threads on this subject. In fact about once a week someone gets on here asking the same question as you. I don't know the answers but I know there are numerous threads on it.