[Kat]

Just from the pic, the frame rails look to be ok.. a good media blast of the aera would be able to tell how bad the damage is. You might be able to get away with a tail pannel and call it good. As long as you media blast the aera well and prep it well.

Oh and screw POR15 if you are going just to do that, might as well get some epoxy ppg paint and spray the area before you close it back up