Have you actually had the PTO off of the transmission ???
Are you sure no gears are engaging the transmission gear?
I'm wondering if it is mounted upside down on the transmission. That would explain the output shaft to nowhere.
All I really know about them is that I just found a decent looking PTO in the junkyard and bought it cheap and brought it home and it fit right onto my SM420 transmission. I have no idea now what brand it was since I sold the truck 20+ years ago. It seems to me like it was made to swap the gears around and shift the output rear to front (like for a winch or something) but I think it worked for me just the way it was.
I cut a slot in the floorboard and bolted a short piece of angle iron next to it. Then a hole in the vertical flange for a pivot bolt for a flat piece of steel about 1" x 18" for a lever, bolted directly to the shift rod in the PTO at the lower end. Worked great, unlike the log loader that it was powering.
It can't be that difficult if even I managed to figure it out
