Your transmission on the floor? Did you figure out why you lost reverse? Gears/clutches or valve body?
The fluid is brown and stinks to high heaven, some of the sprags show a lot of heat, and the clutches show a lot of wear. I've been doing a ton of reading and it seems very common to lose reverse. One opinion is the reverse solenoid piston is too short not allowing enough pressure causing it to slip. Smoked clutches allowing them to weld themselves together is another.*
I also found one I'm leaning towards but can't prove because it's apart. The plug was pushed in enough it was leaking fluid. From what I'm reading if fluid gets in the plug (and it was) it will cause it to short out and default to only forward gears, even in neutral. From what I'm reading, I could have cleaned the plug and it might have fixed the reverse. Regardless with the smelly fluid, it definitely won't hurt to rebuild it. Or now that I know how easy they are to pull apart, I could swap tailshafts into another donor if this one has too much ruined.*
I'm trying to study all I can and learn how to work on them. Always been overwhelmed by it, just like doing gears. But once you grasp the basic concept of it, people say it's not that bad with the right tools. Just keep it clean and check your tolerances.