Use the MSDS y-pipe. IIRC, it provides more power than MSDS headers in conjunction with SHO y-pipe.
I had MSDS headers with SHO y-pipe and it was RASPY.
Went to the MSDS y-pipe - rasp was almost gone and felt more powerful.
I can't remembe exactly, but I think you need the stock cat with the appropriate number of bolts (2-bolt IIRC) to bolt up to the MSDS y-pipe.
HTH
