The stock 3 speed with overdrive is a T85. They came in an early model with a 4 bolt side cover that didn't have a synchro first and a later(post 1966) 7 bolt side cover that did have a syncro first. I have the later one that I put in my 1966 GMC with a 305 V6. The overdrive is a .7 to 1 so it will reduce your RPM's by 30%. Here is the original literature on the early model T85:

http://speedprint.com/deves50/overdriveindex.php

Read all about it!
Regards,
Steve