Shoulda gone to SOP! Philip and TXS tore apart a 518, which is just a 727 w/ overdrive. They gave lotsa tech tips too. Like, check the input shaft thrust before taking the trans apart, then you know if you need to get thrust washers. And, put air pressure to the clutches and stuff before disassembling the trans. (after the VB is out) Shows if you have a bad seal before you disturb anything else.
If you're going to put a Cummins in front of it, change the planetarys to all steel units. The aluminum ones suck!
Matt, it's not that hard, especially with a manual.
Good luck!