I found that the center flange for cylinders 3,5 and 4,6 would warp from heat and blow out the top of the ports. The fix is a price of 1/8" by 3/4 wide by about 4 or 5 inch welded to that flange, laying on the tubes to stiffen the header flange. If your flanges are already warped, build up the gasket side with braze and Sand flat.