First and foremost: Soak everything in penetrating oil. A LOT.
The right stuff to use is Aero Kroil which is a little hard to find. In a jam, Liquid Wrench will suffice. Do not use WD-40; it's not penetrating oil. Really. It's often misused as that, but we can't afford to fool around here. Those bolts need to come off without snapping.
You need to soak every bolt associated with this assembly. And this is a big assembly. Soak them as often as you can.
This is a huge assembly comprising both cats, 0-4 oxygen sensors and a lot of pipe.
The assembly connects to this exhaust manifold, the other side's exhaust manifold, and a flange at the back that may go to another catalytic converter or simply the exhaust pipe. Soak all these bolts.
Also, search the pipe for oxygen sensors and soak those too. It'll help a lot if the oxygen sensors can be removed intact, as they are a matched set.
There will be oxygen sensors both before the cat and after it. Since you have 2 cats, I presume 4 sensors. The pre-cat sensors may be on the exhaust manifold; if so disregard.
Best option: have a pro shop weld it.
The reason is this whole catalyic converter assembly is costly - about $300 for the part. (possibly a lot more in the 12 states that follow California emissions). But honestly, that's really not bad at all for that huge assembly with two whole cats. That reflects the popularity and huge volumes of parts sold for GM C/K families.
Back in the old days, exhaust pipes were the first thing to rot out on cars. You'd maybe get 30,000 miles on an exhaust pipe (in the snow belt). What changed is they started making them out of stainless steel. No, it's not shiny and looks corroded, but boy howdy they last a lot longer!
So this is not a thing you could possibly DIY-weld. Even if you succeeded, your welding would alter the steel so it isn't stainless anymore. And that means it would rot out quickly like in the good old days. So you'd be right back to buying another one.
However a shop has a reasonable chance of being kitted and skilled to do the correct stainless welding.
Since they would need to weld this on the car for fit, I recommend aggressively cleaning any leaky-oil mess from around the area. They will not want to weld around that.
Plan B: Unscrew those screws and replace the whole cat assembly.
Normally I send people to a Pick-n-Pull yard for spare parts. Cheap, and you get a lot of "free practice" doing the work on expendable cars. Unfortunately that won't work here, because the yards chop off the catalytic converters. They'll have the ends of the pipe, just a Sawzall slash where the cat proper should be.
You might be able to get the little chunk of pipe ahead of the cat, but really, any exhaust shop will stock that.
You might ask full-service (THEY-pull-it) wrecking yards if they can sell you a used cat assembly flange-to-flange. However the government discourages used-cat sales, both for clean-air and anti-theft reasons.
Your cats proper are probably fine if you're not getting a Check Engine light. 1996+ cars have oxygen sensors both ahead of and after the cat, so they can detect a failed cat. That's why I want you to keep the oxygen sensors as a matched set - they work together.