So far the worst transmission i ever had to wrestle was the NP 435 in my 70 D100. For some reason the input shaft wouldn't slip in, even after I did that plastic spline dildo thing to it.
I had to get under the transmission and push it up with my knees and try to push, I probably looked like I was in labor but that little basterd just wouldn't go and it was heavy as hell. Finally I got a bolt started but still had to fight it all the way.