I originally wrote this for the ecomodder blog, but nothing is being posted and I haven't heard from the admin in quite some time, so I'm giving yall the information here.
I recently swapped out my transmission in order to get the power of overdrive, and encountered all sorts of unexpected problems in the process (due to having never done a similar project before) so I did an extensive write up to ensure anyone else who does the same or similar in the future can avoid learning them the hard way.
The Flamboyant Introvert: Adding an overdrive (BW T-19 to ZF-5 transsmission swap)