[McMurphy]

Just so you know, the PO of my rig used that foam in the door rockers, literally filled them, and the cab corners with the stuff.
What we found when we pull the foam out of the corners and cut out the rockers was that the foam retained water, and was rusting those areas from the inside out.

In my truck it was being used for winterization insulation and filler.

There is a method of correcting oil canning using a propane torch and wet rags. All you do is heat up the edges of the area of interest, and fast cool it with the wet rags. You work your way around the area giving you trouble and towards the center until the metal has shrunk enough to regain its surface tension.
Pretty good YouTube video on the process if you want to look it up.

Looked fairly easy, though I have not had to do it yet.