This can be addressed with some work on the weld bead around the port on the headers. I've noticed that this area, especially in cheap headers, needs some work. Widening, flattening, and equalizing the ports between them all goes a long way towards creating better sealing surfaces. Weld em up, grind them smooth, take some time to make them level, then cut the band between all the ports to allow each tube to move independently of the others.