How to keep a panel popped out

[dflarsen77]

I’m doing body work on my 70 C10 longbed fleetside, working on the bedsides. At the rear of one, the outer panel is dented and popped in. I can pop it out by pushing from the other side but it wants to pop back in. I’ve been doing studs and a slide hammer around the outside but it’s only helping minimally. Do I need to cut the whole area off and work at it that way? I’m new to bodywork if it isn’t obvious.