It looks like something hit that area and dented it like that.
What gives the frame its beam strength is those upper and lower flanges. Someone I think mentioned drilling a hole at the end of the crack, good idea helps spread the stress and stop the cracking process. It will be hard to fix that and hide the fix. Personally if it is going to be a driver, and not just sit in a museum. The option I would use would be to properly weld the crack with of course straightening the flange etc. then find another frame section that is smaller to fit inside that frame rail, and bolt it in, and weld it, just look at the way outfits like Kenworth double frame their trucks. That will give you the strongest repair.
