I hate to say it, find a whole frame, or scrap it. Since you said your truck has some history, I'd try to find a decent frame, I'd sell ya my 8" frame, but it won't be available for at least a year. You could get a whole rear section off a gasser, Z cut it from the cab back and splice it in, with some fish plates. Clear as mud?
Try to get some structural integrity back, even if you have to use some C channel or L shaped stock to reinforce what you got...