I'm not sure what else you'd like to know. The tech library here says that your stock rear axle on your 92 is 56.5 inches wide. Roughly measuring the Cobra IRS, it looks like it's around 63 inches from wheel mounting surface to wheel mounting surface. Like Don said, it's about 6 inches wider than what you have under there now. I'm assuming you'll want to run some pretty wide rear tires when it's all done, so you'll need to figure out how you want to mount the IRS, determine the tire width you want to run, and you may need to modify the bedsides depending on the offset of the wheels you choose.
If you mount the IRS with something like the Truckirs.com kit, it will force you to run the wheels further out. Doing it like the Trucks! Guys did, and removing the stock mounting ears will give you much more space under the bed for tires, and gives you more options for wheel offsets, but will require more measuring/fabbing on your part.
The stock exhaust won't work behind the muffler, so you'll have to do something else with it. Dump it, run it out the side, reroute it under the IRS dif, etc.
I haven't checked the driveshaft to see if the pinion flange Will bolt to the Cobra IRS or not. That's something that I'd address down the line after the hard work of installing the IRS is done. Custom driveshafts are fairly cheap/easy to source.
You'll also have to decide if you're going to use the stock coil springs and shocks, or convert to coilovers. Coilovers will increase the cost, but mean you only have to fab 1 set of mounts. Keeping the stock coils and shocks means you have to fab spring pockets as well as shock mounts, and have less adjustability than the coilovers would.
A lot of this is going to just be up to you, and you'll have to figure it out as you go. There's no recipe for this, or easy-to-follow "how-to" in the tech section. If you're serious about it, do lots of planning, lots of measuring, and fix the issues that come up as they come up. I'd start by setting the wheelbase and ride height you want. Mount the IRS cradle where it needs to be for that, and everything else will be easy from there. Route the exhaust where you need to so that it's out of the way. Pick the wheel/tire that you want, and adjust the body to fit (bulge the bedsides, add wheel flares, etc).
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