Nancy
If they were saved, any plaques that were on the bridge may have ended up in one of maybe three places. They could be in the local historical society (where they should be); collecting dust in a highway garage (I have seen this many times!); or on someone's mantle (most common place). Likely the rest of the bridge was scrapped.
I am pleasantly surprised that this bridge is considered a part of your Historic Bridges of the U.S. My grandfather was the engineer or part of the crew of engineers that built this bridge. I have an old photo of it when it was new. Before my mother died in 2000, she had heard it was going to be rebuilt and was wondering how she could get a piece of it, such as a bolt or a chunk of iron. I remember wandering about Morris, looking for a historical society or someone who would know where it's pieces were taken, especially the brass plaque that may have been attached to it. Does the state keep this stuff in a museum somewhere? It would be nice to get a picture of the plaque, at least. Any information would be greatly appreciated. I dabble in genealogy, it would be a nice addition to my 'historical' photo collection.