I had a Downey header on my last rig do the same thing. My Thorley hasn't cracked yet, but it hasn't been hit, either. I don't think it's the fault of the header, just the way things are. You're bolted solid on one side, the other side flexes, any hit and the hangers are gonna try to take up the slack. take a big hit, and the hangers hit the frame and the truck keeps moving; somethings gotta give.
