Another is to remove one end piece from the tailgate to be able to insert an anvil (or dolly in the body shop vernacular) inside the top roll and work the dent out. I had a bed rail that was flattened and used a large ball bearing that I welded to an adapter screwed onto a slide hammer. My dent wasn't as deep as yours. Here's some pics of that approach. You'd then need to plug weld the end piece back on. Either way, it'll be a lot of work.
The 16 gauge metal will be pretty hard to pull out with weld studs and slide hammer, but it might eventually work.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
