I picked up a factory dual exhaust off of a 4th gen (somebody who upgraded theirs, craigslist). This is my attempt to clean up this nasty resonance that GM didn't take care of, hopefully by changing the frequency a little. If you rev it unloaded, it does not sound very good at all, but under load sounds very good. The rattle is still there but not as bad. My next step is a resonator up stream...

Old exhaust:

For easier manipulation, I cut it right here now its ready for clamps:

I kept the V6 pipe until the muffler (so I can retain the hanger) I had to close the gap by building up the weld on one side:

Tweaking the position:

Video of open exhaust:
Open Exhaust

Video of completed duals: