I was out yesterday doing more searching for hidden rust and putting new rustproofing on my '99 Ranger. I found out my truck has a hole about the size of one's fist in the steel of the bed, in the front driver's side corner area. I've never noticed it before, as my truck has the OEM plastic Ford bedliner, and a previous owner also put a specially made rubber mat on top of that. My truck's the stepside style and has the fiberglass bed sides. The outside looks great for a 22 year old truck. Could I possibly patch the floor of the bed by knocking out the rusted area and welding in new sheet metal? The brackets across the frame rails that the bed floor sits on are all in excellent shape. 

I can't see the damage from above, as I can't figure out how to release the liner. I changed the bed rail guards the other day but couldn't figure out how to release the liner after the rail guards are off. Do the four tiedowns in the corner of the bed hold the liner to the bed as well? Those are the only other thing I could see holding the liner in.