WrestleMania 3 Questions

How the hell did I never hear about the Orndorff back up plan at Mania??
I always wondered why he wasn’t on the card.
Can you shed some light on the subject. 
Was he feuding with anyone at the time?

There is no way that unless Andre died right before they would have done something else right? 

175,000 people came to see Hogan bodyslam the 740lb giant!!!!!

​Technically he was just finishing up the Hogan feud, but otherwise, no, he didn't have anything going on.  He did the babyface turn and started a program with the incoming Rick Rude a little bit after WM3, so he was mostly in between stuff anyway.  Plus Orndorff himself was in rough shape after the Hogan feud and was pretty much told to hang it up due to the extensive damage done to his neck that needed surgery.  Andre himself was wrecked and there was serious concern that he wouldn't be able to even move enough to do the match, which is why the Orndorff plan was kept around, but as you say, he would have needed to be dead that day instead of the next day like really happened.  ​