Pull the bumper and intercoolers off. It's only about 15 minutes of work. You'll be able to access the nut and O2's much easier. With more access to the nut you can hit it with an impact hammer or a chisel and hammer. Hit it on the side of the nut in the direction you want it to spin. The impact hammer or chisel will dig into the nut and either cut it off or spin it off. I had the same problem and a hammer and chisel got the nut off no problem. I didn't read the whole thread but you completely destroy the nut in process so you'll want to order new ones.

I recommend placing a jack under the muffler system after you get that pesky nut loose so it doesn't just fall. It is pretty heavy!

Didn't realize you didn't pull the bumper. You should as it will make the install much easier too. Takes about 15 minutes - super easy.

I'm going to pull the bumper in the morning. Was hoping it was going to go a little smoother than this but with the bumper and intercooler out of the way it should make life easier.
My last resort is using the chisel. I don't want to risk damaging the stud and I have all new hardware

Thanks for the input !

FWIW, once you get the bumper off you will have plenty of room. IMHO you won't need to pull the IC's.

Also I rented a metric stud puller from autozone. It made pulling the studs out SUPER easy.

That is just in case you mess up a stud.

With the bumper and heat shield off, off you may be able to get one of your wreches on it approaching from the rear.

I had the exact thing happen to me when I did mine a couple months ago. It seems that it is always the last nut... Definitely take the bumper off as others said, sounds like you have already done the pb blaster soak. What I did was hammer a quality socket on (don't use a cheap one or you will continue to round it), then I was able to use an extension bar on a socket wrench to engage enough with the socket to get it free. It may also be worth it to just buy a curved wrench, which will allow much better access. As others have mentioned, replace all of the nuts for sure.

Got it! Im pretty impressed with how far I threw the nut. Took the bumper off which didn't really give me more access to the nut but it'll help dropping the exhaust.

New issue, all the bolts are gone, 2 hangers are removed and o2 sensors are removed....now there is something hanging up in the middle of the exhaust and it wont drop down at all. Here is a pick of 2 15mm bolts that are right in the middle, do they have to come out as well?

Nope. Just the 8 nuts to the turbo and the hangers. My guess is that the straps are not fully disengaged with the brackets in the back. Even though I could clearly see them, I still had to slide the straps farther to the outsides of car than I thought I would. Make sure you have a jack under the exhaust when you are tugging in case it lets loose.

+1 on what dsa22 just said. The hanger straps are probably still hanging up. Once they are out of the way, unhook the muffler from the cars' bracket by picking the muffler up and away. Keep the jack under the muffler to hold up that heavy muffler.

SUCCESS! Thanks guys
Just like that last nut it took some brute force to get everything loose.

The gasket on the turbo looks fine to me, what do you think?

I wasn't happy with the nuts and washers that I got so hoping to source them tomorrow from Pelican or failing that I will be in Denver tomorrow if anyone could recommend a shop that might have them.
Do you need to put anti seize on the turbo studs?

Installed the 02 sensors on the new CATs and thats all I can do without the proper nuts. Traveling this week so hope to have it finished this Friday when Im back.

Thanks again for everyone's input

I bought some stainless steel nuts from lowes. Why not get them there?

I got M8 125 stainless with regular flat washers cost like 3 bucks for all 8 nuts and washers
I'm afraid they won't lock on and back off. The OEM nuts are slightly tapered and the washers are slightly cupped to apply pressure to the nut so it doesn't back off.
Just thought $3 was a little cheap for what it's doing

I bought some of the wave lock washers.

I figured I needed something I could get on and off a lot.

Just get some lock washers. At least you can start it up and drive it... If one backs out later, then fix them.