There is NO way that these are coming off with a socket. [SIZE="3"]

These are the bolts that are pressed into my 3rd cat that go through the flange on the exhaust pipe you see towards the rear of my car.

Ideas so far:
-torching
-PB blaster
-4" grinder (risk of damaging flanges)
-dremel with grinding wheel

 

If you plan on messing with any of that, just plan on replacing parts.... that aint coming apart and then going back together. You can get anything apart.

 

Sawzall with short metal cutting blade. Will allow you to get in there and should not hurt the flange too much.

 

I already have replacement nuts and bolts in hand. Compromising the integrity of them is not an issue. I want to preserve the flange though.

 

If you have shop air, I would suggest an angle die grinder, with an abrasive cut-off disc; probably about 2-3" size, for maneuverability. I use the heck out of my little Ingersoll-Rand die grinder. Their small, air-powered reciprocating saw might work, (but mine does not impress me.) Seriously doubt you will get anything to turn. I'd also support just using the 'ol "flame wrench" on either side of the cat, unless you're in an e-check state. I'd take off the cat and install a "test pipe" or flex pipe; cut to fit, with muffler clamps front/back. Not a huge job.
Put on your safety specs, for sure.

 

The OEM J-pipe(aka down pipe) is being replaced with an aftermarket down pipe that actually replaces the 3rd cat and mounts straight to the exhaust. Everything is fabricated and ready to go, just need to get these nuts off without damaging the flange on the exhaust side which the new down pipe will be mounting to.

 

Best bet would be the cutoff wheel, if you must salvage the flange. You'll need to grind flush with the flange, then (possibly) use a punch or drill the rusted (remaining parts) of the studs/bolts out. (I would think.)
Having said that, are there any O2 sensors (pre-post cat) you need to observe? If the flange is too rough, after you get things apart, I might suggest going to the local AutoZone and getting a couple "bottleneck" fittings to use on either side of the cat, for the inlet/outlet pipes to fit into. Tack weld the fittings to the cat, and use muffler clamps on the pipes.

 

You said your replacing...CUT IT OFF...you can buy any size flange

 

Cut the cat off and sell it to a metal recycler.$$$$$. You can then drop the exhaust off the hangers to get at those nuts with a cut off wheel.

 

Hot wrench used by someone with a little bit of experience using same, NO PROBLEMO!! Used discreelty only the nuts will be gone, the ratty bolts can then be hammered (ever so lightly) out. BTDT.