The technical name for the 3 speed with overdrive transmission is T85 or T85N. They were two versions used in GM's. The first was with a non synchro first used from mid 50's through 1965 (four bolt side cover). The second was with a synchro first used from 1966 to early 70's. The second version is the one that I just installed in my 1966 GMC with 305E V6. Do they work? Yes, I am very happy with mine. They are basically a bolt in swap for the trucks with a factory 3 speed (except I used a floor shifter). Here are some links where you can read more about them:
http://www.classictruckshop.com/club...5n/rebuild.htm
http://www.classictruckshop.com/club...cts/how_to.htm
http://speedprint.com/deves50/overdriveindex.php
The transmission has a .7 to 1 final ratio. That means a 30% reduction in RPM's at highway speed. They shift "automatically" at 30 MPH into overdrive through the use of a governor switch and an electric solenoid. The trannies pop up on Craigslist and E Bay periodically. I bought mine from Craigslist. The only problem that I had was UPS smashed the governor in shipping and wouldn't honor the shipping insurance. If you have one shipped, you should have the seller remove the governor and the solenoid before shipping since these parts are hard to get and are damaged easily in shipping.
I hope the above is helpful.
regards,
Steve
New Orleans