I think on the last pic, I would extend the horizontal cut line all the way off the end at the fender opening to include the hole. There appears to be some pretty deep pits at your vertical cut line that may cause some blowouts when welding back in. On a flat panel like that, you have to be sure to do the dot and planish weld method to keep it flat. It's pretty easy to get it warped. Review MPandC's thread on MIG welding up at the top of the forum.
Otherwise, your plan looks good to me.
What if I tack weld a strip of metal behind the original bed wall? That would help avoid warp, right?
