where did you get the flanges from? I like the unified-flanges design, I've seen a lot of ill-built 321SS turbo manifolds hang the turbo way out there, creating a ridiculous moment on the head flanges and welds. Even heavy wall thicknesses and good welders out there can't keep them from cracking. High turbo manifold temps and constant frequency stress reversals is my guess as to the main factors. I suspect weak motor mounts may be expediting the situations where i've seen this hapeen.
Don't have any hard evidence/enduro tests on turbo manifolds from our engine dyno though, its mainly been used with NA and supercharged motors.
Oh by the way Ash- make a zemulator for the 240SX crowd! I just might know of a california-based tester for you... :)