Okay, after it was all said and done, the rattle is still present under the same conditions.
I installed the new magnaflow converter with Denso A/F and O2 sensors, had to helicoil one of the exhaust studs back into the passenger side head because the threads pulled out when I went to tighten them, but other than that the job went fine.
I noticed that the rattle is easily replicable at between 45-55 mph, under heavy acceleration (downshifting), and the rattle starts at approximately 4500 rpm and continues to get louder the higher the RPM's go. The car also seems sort of "sluggish" and underpowered at that RPM and gear (4th I'm assuming?) but readily picks up speed in any other gear and so far I've not heard that rattle at low speeds with high RPM's only at 45-55 or so.
Very strange....