Cutting spot welds isn't that much work with the proper spot weld tool. And you can plug weld the new angle piece to the bedside from the outside and finish the welds to be invisible. Again, that's what I'd recommend.
If you decide not to remove the angle strips and replace with new, have your sandblaster concentrate on the gap between bedside and strip both top and bottom. There IS rust under there, and you'll need to get as much as possible out. You might consider using some rust encapsulator produce worked into the gap.
