In reading your post, it sounds like the leak is on the transmission side.
Removed the drive shaft for the pump, should be two cotter keys, slide the collars and the slotted shaft drops out. Then use a square pry bar to remove the drive coupling from the transmission drive shaft - it is threaded on, regular right handed threads. Righty tighty and lefty loosey.
3 bolts removes the seal retainer, stuff a rag up there to stop the oil from running out. Remove the old seal from the seal retainer and drive the new one in. Original seals were very thick, two to 3x thicker than a modern replacement. ONE modern replacement seal should be plenty.
Re-assemble and go to work. Just did this over the weekend on my M.