00 4runner Catalytic Converter replacement?

My mom's 4runner has had a p0420 code for a while and after trying the other possible things it appears to be the cats failing.  It has 240k miles and they seem to rarely last that long in those anyway.  It looks like changing them (ca emissions so it has two) is going to require pulling the transmission, but I want to see if someone might know if I'm missing anything.  The front oxygen sensor plug is on top of the transmission, and I can't fit my hand far enough into the transmission tunnel to unplug it, and just taking the mount out and lowering the transmission on a jack would make it worse.  The bigger problem is that two of the studs on the connection from the crossover pipe to the cat are seized. 

It looks like I would have to just snap the studs, pull the transmission and take the crossover out to fix or replace (hopefully without more studs breaking) and unplug the old oxygen sensor and plug the new one in, then put it all back together?  That seems like an enormous job when it would be a matter of 30 minutes on my old Subaru, especially when it would turn into a "while I'm in there" clutch replacement too because I don't want to take it apart twice.

0420 is almost always a failed cat, sorry to confirm.

Question, though... do you live in CA, or is it just a CA emissions truck?  Here's why I ask:  Nobody is going to check either way.  I'm the biggest tree hugger you'll find, but in all honesty, many times the CARB EO number stamped on a CA-only part is the same exact part that they charge 3 times more for than someone buying it in KS.  There aren't very many catcon manufacturers in the world, and it doesn't make sense for them to make one that meets CA standards and one that sucks a little more for everyone else... so they make the same part for all 50 states, and just charge a "tax" for CA residents because they had to pay for CARB testing.

They look to see if it has cats, but if there is no CEL, they're not going to contort themselves to verify that it's a CA part.  They'll look at 1) hey, look, it has cats, 2) it has no CEL, and 3) it sniffed HC, NOx, and CO just fine.  4) Here's your passing receipt, sir/madam, have a lovely day in CA.

I think Rock Auto has you covered either way.  I used to buy parts from WorldPAC for exhaust/emissions and you could buy multiple different components.  You could get pretty much anything from individual parts to ordering a "short cat" (which was downpipes, cats, flange) or a cat back, or just single pieces for almost any vehicle.  Walker was one brand, and the other I'm not remembering.

Both a CA truck and live in CA.  I ordered the cats already, the problem I have is that I started the job then realized the transmission has to come out to unplug the front oxygen sensor and to deal with the studs that I am 99% sure are going to snap unless I'm really missing something.  It's registered in a county with no smog so I am considering ignoring it until it needs a clutch too, the only annoying part is that it goes from 17 mph to 10 with the check engine light on.

I wish I had a better idea for you.  The only one I did was a 1998 49-state 4runner, and it was far simpler.  I was able to replace the cats and the 02s without pulling the transmission.  It doesn't really sound like Toyota thought this one through, unless there is an access panel under the carpet.

I'm really not sure about the front oxygen sensor plug, the best I can guess is that maybe someone put it in the wrong place when putting the transmission back in and it's caught on something or routed the wrong way.  The crossover pipe issue is just cause the studs are seized, and if they get broken off in the crossover pipe then that requires taking the transmission out unless I could figure out some other way to bolt it back together.  If the nuts would come off then it would have been an hour to swap both cats.

My best suggestion is to torch the threads until they're hot enough to melt a candle or a crayon.  You want them hotter than just melting, but not hot enough that you have a huge sizzle or flame.  Just a little smoke is perfect.  That wax will draw right into the threads and give you your best shot of not breaking.

I like to torch the male threads, melt the crayon into it, let it cool, then heat the snot out of the female threads before turning left/right.. kind of like you're tapping threads.  Half turn one way, quarter turn the other, half turn, quarter turn. Repeat until they're out.

Then come back and curse at me because one broke anyway :)