Good to hear on the reasons for the writing etc. IDK on the rust ... that input shaft should have had a layer of oil on it from the trans fluid that flowed through it. If that's rusted, is the pump shaft rusty (goes through input shaft) and bigger IMO is .. did the pump vanes rust, did parts in the valve body rust.
Knowing that transmissions sit in junkyards and on shelfs for years w/o signs of rust, makes me leary of this one. I mean.. in my garage (Connecticut) I've had them sitting on the floor next to the car that comes in wet, frozen in snow etc through all weather in the course of a year and ... never seen a rusty input shaft like that. Hence my thoughts on it. I have a trailer with trans parts in it .. they've been in the rain and the only ones with rust are the ones sitting in a puddle of water.
