You must completely disassemble a TH400 to change the output shaft. At this age it could use new rubber piston seals, even if not worn much. Using the long tail means moving the trans support back, as well as shortening the drive shaft, check your frame. See differences between years at album BUILD A SWITCH PITCH, even though yours isn't SWP, at:
http://s93.photobucket.com/albums/l71/bcroe/
(that is a lower case "L"71, not an upper case "i"71)