I know where you are coming from. I'm on my second header and it just cracked (right at the bend) the end of last year. At $37 a pop, it's not cheap to keep replacing. I'm having someone weld/braise up the crack in the second one. What I plan to do is make sure the header/pipe rests PERFECTLY in the pipe craddles, and I also plan to add a fourth side to my motor box to cut down on motor box flex. It was also suggested to balance the hub of the prop in addition to balancing the tips (Although this didn't help pipe #2)

Good luck............Mark