We pulled the tranny out of my girlfriends truck this weekend ('74 F250), and after looking the tranny over, I pulled the input shaft out and it has one large crack down the middle and smaller pitts around the shaft, as well as, rings wore into the shaft from wear and worn bearings. The tranny is a C-6 supposidly out of an LTD or some sort of car. They put this in 4 years ago when swapping a 460 in. Of course i'm changing it to a truck C-6 and factory 460 install parts. But what would crack that shaft? She had a problem with the fellas putting the motor in before causing a bind between the motor and tranny. This was because of an aftermarket flexplate and stall converter. Would that cause it or torque of a previous engine? It also has a cracked off mount on the bottom of the tailshaft where the car mount would bolt to. Any ideas?

 

I'd like to know myself. I've cracked 2 input shafts on an NP435.

 

what cracks input shafts on C6s usually some idiot using a hammer on it (I am not kidding either) look at the ends and see if it looks like it was used for a punch or damaged.
np 435s have always had some problems with snapping input shafts when used in jerking heavy loads from a stop, like jerking a loaded wagon full of grain out of a field almost always when used behind the 390s or a 460 but I have seen 360s 300i6s, and some small blocks do it too.

 

Excess vibration is the leading cause,usually caused by busted snapring on the input.