Headers break because of movement. You have to figure out what is moving and make it stop. Soft mounts on the engine allow the engine to move - very bad for headers. If the engine mounts aren't stiff enough or the engine box isn't stiff enough then the engine can move. Next - figure out if the muffler can move. Mount the muffler with front and rear supports so that it cannot move. Headers break because that big long muffler acts like a lever arm working the header up and down. A steel header will last longer than aluminum because it can resist fatigue better, however if you can fix the problem mechanically (eliminate relative motion between the header, muffler and engine) you will be better off.