Since you asked, here’s my story: I bought a Borg Warner R10 overdrive that was in a Model A torque tube for my model A. It came out of a running driving Model A. Sadly, the coupling connecting the OD to the pinion stub shaft has disappeared. I now know more than I did when I started this thread. What I have is the BW OD from a Ford, with a 16 spline, 1 5/32” diameter shaft. And I have a 6 spline pinion stub shaft form an Early V8. The overdrive itself was (unlike most of these conversions) was set in the back end of the torque tube so that there was essentially no rear drive shaft, the od connected direct to the pinion shaft. What I think was done was the the slip part of the slip yoke (yoke removed of course) welded to a 6 spline V8 drive shaft, creating a coupling/adapter.