Full disclosure: I haven't done this swap into my 87 K5 yet but I am finishing up a 6.0/4l80 swap into an 89 ramcharger, so I'll share what I've learned. As far as the wiring, do you have the intact wiring harness and computer from the 6.0/80? Do you want to reuse that or use an aftermarket harness? There are advantages to both, namely time vs. money. You can rework the harness yourself using the diagrams at Lt1swap.com. Or you can buy an aftermarket one and it's plug and play. Note that different harness builders place the PCM in different spots. Some put it inside the cab, some keep it in the engine bay similar to OEM. If you've got a preference, keep this in mind. When you take the original engine out of your K5, label all the wires as you disconnect them and lay them aside. Don't cut anything yet. Less than half of the wires coming out of the bulkhead connector will need to be modified. The majority feed lights, wipers, etc. so you don't have to touch them. Here's a link to a pretty helpful video:
As far as the above question regarding the transfer case, you can use the 208 that's in your k5 now, you just need an adapter plate. Novak, Advance Adapters, and many other places make them. There are lots of good vids on the swap and parts on the aftermarket to make it smooth. Keep in mind frame clearance when you think about headers especially with aftermarket engine mounts. I believe the 87 has a baffled fuel tank so you should be ok to just put in a high flow pump and regulator and your fuel system should be fine. Double check to be sure. Remember that if you change the drivetrain placement a lot your driveshaft lengths will have to be modified, so try to get it as close to the original placement as possible to save some cash.
Remember that it won't crank until you get the PCM flashed to take out the anti-theft stuff. You're going to run into a lot of problems that you didn't see coming but nothing that can't be overcome. Keep working and you'll get there. Hope this helps.