i would just put 3 or 4 tig weld beads and give it a go that way, i can't imagine the pain of keep taking it off and send it for replacement - I did that on my camaro driver side exhaust header, they are known to crack between cylinders, didn't have a problem after that - it is cast so that's even worse than stainless, can't imagine a good weld there to crack again - from what i understand it cracks on the 4 in 1 collector between the pipes, most likely due to the welds that pull apart the collector more since they are thicker and more rigid, tig would also help normalizing like was mentioned before, just use stainless rods (i think i used 308 on mine if memory serves me well). Oh, and don't weld the crack itself, weld a couple of beads on each side of the crack and then put a bead between them, don't let it cool and put another 2 or 3 over the initial ones to cover the width and tie into the collector again and should hold forever.