I have been messing with BW overdrives for over 60 years and it is still my favorite transmission. I prefer to have them behind a BW or Saginaw transmission, but they will do wonders behind a SM 318 with 3.89 or 4.11 rears. It doesn't make any difference if you have a chevy with a Muncie with an OD behind it or a BW transmission with an OD behind it in a Y-Block. They are really fun to drive. Very little goes wrong with the OD as long as you keep oil in them. I have driven them, torn them up and rebuilt them. I still have a lot of spare BW parts.
https://photos.app.goo.gl/xaJzGqqyzwPs2GqH7Maybe some day I will be rebuilding them again.
Her is a link to a service manual.
https://documentcloud.adobe.com/link/track?uri=urn:aaid:scds:US:b1d2777f-0f56-48fc-8922-a45fdbb8cf51
As for the Plymouth, it could have a BW OD or it could be a Fluid Drive. My father bought a new 1949 DeSoto with fluid drive from his brother. I hated the thing, but the transmission outlasted everything else on the car. My cousin still sells Chrysler-Jeep-Dodge. If you can't get under the car to look an OD will have a lockout handle under the dash and usually have overdrive written on the trunk. Some of the parts interchange between different R10 ODs and some don't. I usually just compare parts.
