Guys like you and John who weld with quality open my eyes to possibilities I had not thought of because you can remove and reinstall pieces I had not considered. Removing that end piece to see inside is good thinking Frank. I appreciate you sharing what you found in there; I would be afraid of what I would find. Again, my mind does not go there naturally because I would have to weld the piece back, and I have seen too many of my welds fail. Simply put, I do not have a lot of experience or training in that area, but I stay at it.
Once when I was repairing a body tub I used the original inside rocker panels. They were in ok shape plus I discovered the rocker itself is open on the back end in the wheel well, so I did what you are doing with the frame and cleaned it out and made a mop and put some rust inhibitor paint in the channel. Going back to my chemistry story, the free floating electron that is picked up by the incomplete iron atom is a loose oxygen electron, making iron-oxide.
Peace out steve