Am I missing something obvious, or is there a special style tool to remove the nut bolting the catalytic converter to the exhaust manifold on a 2000 ls (driver's side)?

The one nut is easy to get to and remove, but the second nut seems to be virtually impossible to get loose (or tightened).

same issue here. No one seems to know. any advice

i had to use (from the bolt to the gun) a XXmm deep well socket, wobble bit, short extension (3"-4"), wobble bit, long extension (8"-10"), impact gun.

biggest PITA i've seen in a while.

also had to soak them in PB blaster every couple of hours for a day.
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As I said in the other thread... from certain angles one of the bolts is difficult to get to.

I would just remove the pre-cat O2 sensor.

also, make sure the exhaust is support, as it will remove some of the weight from the nuts and make it a bit easier to get them off.

You may want to try hitting it with a breaker bar to break it loose before trying with even an impact wrench. when I first removed the exhaust about a year ago, I found that my impact gun was not strong enough to loosen one of the bolts on both sides. SO I used a 2 1/2 foot breaker bar, and it seemed to do the trick.

PB Blaster or Liquid Wrench certainly helps as well.

I used a Stubby Wrench on one of the nuts. I'm sure it was the same one. It came off relatively easy. It surprised the hell out of me. Like mentioned above use some PB Blaster or something like that and let it sit for a while. Add some heat as well if necessary.

nothing special - just a tough angle...

Flowmasters Quik?

yep - 40s