Borg Warner Overdrive Problem

[rotorwrench]

It sounds like a slip yoke spline to me. The 16-spline was fairly common on Ford manual transmissions and some GM power glide transmissions. Others may have used it as well. My old Mercurys use the 1M-4841 as well as Fords through 1953 and it is similar to the old 21C-4841 that carries back to 1942 for light duty commercials.

 

[Bob C]

Here is another company that was building BW OD. http://www.godevilgarage.com/
Maybe get a couple of slip yokes with the needed splines and weld then together or
there is a company that can make you one. https://www.hayescouplings.com/splined-adapters/