Just a tiny bit of input from my limited knowledge. I believe part of the conversion of the Saginaw 4 speed included modifying the main shaft length and splines to work with the new tail shaft ball collar design and output yoke. If you're contemplating using that other 3 ring Saginaw 4 speed, you'll either have to swap the existing modified main shaft to that transmission, or modify that main shaft the same as your current unit for things to go together and work like they should. Might be a bigger bite of the apple than you want to take on.
Just food for thought.
