In the 70's I built a Saginaw 4-speed overdrive by putting the OD from a Saginaw 3-speed on the 4-speed. This was a long time ago and I don't remember a lot of the details.
I recall that making the floor shift linkage work right was one of the bigger challenges.
If I were doing it today, I'd look for the cable based shifter from something like a Citation. I don't know how available those are today, but when I parted one out years ago I was impressed by that shifter.
In reality if I wanted an OD manual in a Stovebolt I'd do a T-5 conversion.
