If the bottom of the bedside was just bent then there is likely little or no stretching in that area. Although depending on how it was bent and the exact constraints of the panel you could end up with some stretched metal.
I think your best plan of attack would be work the metal off dolly, the objective here is to move the metal around and not stretch it. Hold the dolly behind where you don't want it to move and hit where you want it to move. The key is light taps if you start thrashing away you will not be satisfied with the results.
If you get to a point where you just cant seem to tune it in perfection you likely have some stretched metal and the best way to shrink and automotive body work is a shrinking disc. Do a search here and on metalmeet.com regarding the use of a shrinking disc. You could use a torch but it doesn't allow for the same controled shrinking that can be easily achieved with a disc.