It sounds like the transmission cannot be saved.
You would need a transmission prior to 1993 as that's when the rear extension housing changed to an internal bearing type. Best to check it matches the same rear housing as yours (non bearing)
Also have the entire crankcase ventilation assembly changed (oil trap, flame trap, flame trap hose, and the hose from the flame trap to the intake pipe
If your engine hasn't been maintained which it sounds like it hasn't, the oil not being changed often would lead to a blocked oil trap, which would then lead to more oil leaks as the engine cannot breathe properly