Years ago I pounded a piece of aluminum to the shape of the ridges and took it to the junk yard. Bought a Dodge tailgate for $30 with the same pattern, cut out the bad and welded in the new. Even had the slope down to nothing in the very back. Now the vendors, some of them, sell patches for the bed. Depending how bad it is, a cheap welder and a donor tailgate could be cheaper route. Bed liner is great for disguising bad welds.
Or, you could strip everything, buy a spot weld cutter and go to town. I just wanted to drive.