Thanks everyone for the feed back! Looks like I'll do three cross members, install heavier duty shackles, and go with wet bolts/bronze bushings.
It looks like it might be better to weld cross members directly into the hangers rather than welding them directly to the frame (like the MorRyde X factor system)??? If that's the case then would have to use channel beam rather than a box beam. Would it really be better to use channel beams between the hangers or use box beams welded between the new sub frame?