Sharing all the info I have on the Muncie SM318 trannys. Rebuild kits are easy to find on Ebay or you can buy the pieces part by part at your local store and make your own gaskets. See me rebuild mine
starting here. Hijinks included.
The OD on some of these 318s is supposedly a big difference in performance for our trucks, allowing them to get up to 68mph at top end and get much better gas mileage. I might be falling into one here shortly. You have to have a special drive shaft made or have yours shortened to make it work, but then you're good to go.
They work by kinda putting a second transmission behind your first one. The second tranny is run by an electric solenoid that pushes it into gear when you want to up-shift. You get up to 50 or so with your normal tranny, hit a button, and it shifts into "fourth" gear and you can continue on.
Since its two different trannys you actually have six gears now, but mainly theyre used for a 4th gear. Here's a wiring diagram I found for the OD unit
You can buy a repo solenoid here:
http://www.fifthaveinternetgarage.co...counter_3.html
Attached is scans of the SM318 rebuild manual, OD operator's manual, OD service manual, OD rebuild manual,OD wiring document, and OD tips and tricks.