Frost:
Which 3 speed do you have? Does it have 4 bolts on the side cover or 7? The first would be SM318 and the second would be SM326. The Overdrive 3 speeds come in the same variety- 4 bolt side cover and 7 bolt side cover. I would get the same version of the overdrive as your original, if you want the easiest install. The 4 bolt side covers are easier to find in overdrive since they were used from the 50's to mid 60's. The 7 bolt side cover overdrives were only used for a few years from the mid 60's. I have the later version with 7 bolt side cover. It is a great tranny. The main difference with the later model is that they have a synchro first gear. The overdrive parts, particularly the soldenoid and governor are getting a bit hard to find though. Make sure when you buy one to check to ensure these two parts are good. The trannies come up pretty often on Craigslist and Ebay,but they are heavy to ship. I usually use Adhuntr.com and you can search the whole country's Craigslist ads at one time or just a state or region.
Here are some links that may be useful:
http://www.classictruckshop.com/club...5n/rebuild.htm
http://www.classictruckshop.com/club...cts/how_to.htm
http://speedprint.com/deves50/overdriveindex.php
Hope that helps.
Regards,
Steve
New Orleans