I'd be interested to know how the saddle/frame key issue was resolved. It is my understanding that 476 ran for several years with loose or missing saddle/frame keys which allowed the saddle to work in the frame, which wallowed out the tapered bolts holding the smoke box to the saddle. Of course having a rotted out smoke box bottom would make this problem happen much faster.
The SP 2248 I ran in Ft. Worth had a case of a loose saddle key. Nothing seemed to be moving, but the key kept coming loose. Both the key and slot were quite visibly worn. Finally out of desperation I had this big guy who worked with me, take a big sledge hammer and drive it home. I then drilled and tapped a 1/2" a hole where the keyway and key met and locked the works together with a bolt. It wasn't the most graceful and correct way to deal with the problem, but it kept the old girl on the road.