Anchor the front of that beast some how, space and spot weld some skinny bolts down that back ridge that bends inward and hook you a come-a-long onto the bolts and start cranking. I would start pulling about 3-4 inches from bottom first and go from there. Just watch how the metal moves as you pull and it will let you know how much and where to pull next. You would be surprised how much of the damage will straighten doing this method.
Best of Luck, James