Not trying to be mean, rude, start a flame war, or otherwise be a pain in the ass
Based upon what the cars themselves have to say, I believe that the idea that the idea that the 1920's rebuild process involved building entirely new cars using some old hardware is an old wives tale, perhaps even started by the Rio Grande.
I have done major work on 4 300 series boxcars and looked very closely at least 30 others and have yet to find one that does not a lot of original wood parts in its wall structure and frame. Even the picture from Railway age in the posting above clearly shows the rebuilt car with a mix of old and new wall timbers, with most of them being old.
I believe that the Rio Grande reused every part of the original car that was possible, if it did not need to be replaced it was not. Thus, under the new Murphy Roofs and new siding, much of the original structure remained. While there might have been a car here and there that received entirely new wood during the rebuild, I think they were a rare exception and one that I have yet to encounter.
Please note that this statement applies only to the 3000 series boxcars, the high side gons, drop bottoms and stock cars are a different story.
Jason Midyette