Is this the /od you are talking about?
http://www.chevytrucks.org/tech/overdrive.htm It would have the advantage of not having to change trans mounts or drive shaft.
It would be very similar to the '84-88 Vette 4+3 Overdrive. It is a Super T-10 with a 2spd automatic overdrive tailhousing and controlled by computer. The computer locks out 1st over, hence the 4+3 = 7 speeds. The problem with this setup is that the computer doesn't always shift when you expect or want it to.
This site just gives a little info on the od and the wiring for it
http://www.classictruckshop.com/clubs/earlyburbs/projects/over/drive.htm
