Adapting an early Borg-Warner overdrive to "a more modern 3-speed transmission" will not be simple and will likely involve some expensive machine work.

i suggest that you look for a Saginaw three-speed overdrive (a Borg-Warner unit) transmission used in 1965-1970 behind Chevrolet inline sixes and small-block 283 V8s. Using the mainshaft from the three-speed transmission, you can also adapt this unit to a Saginaw four-speed. If you acquire one of these overdrives, make sure that you get all of the parts, most especially the solenoid, which--as i have discovered to my chagrin--is unique to this application.

For a manual of Borg-Warner overdrive operation and some parts, see

http://www.fifthaveinternetgarage.com/parts/index.html

For a Borg-Warner overdrive service manual, see

http://carburetor-manual.com/borg-warner-overdrive-transmission-shop-repair-manual-ebook-p-322.html

God's Peace to you.

d
Inliner #1450