If you have dual tanks here is a diagram of what you need.
http://www.hotrodlane.cc/Instruction.../dualtank.html
Since the LS based vortec is a high pressure fuel system you will need to change the tank selector valve for a high pressure unit. I believe this one will work: Pollak 42-159
You can get an adapter to hang your current transfer case behind the 4l60e to keep your current passenger side drop setup.
If you keep a stock (for the 87) AC system then there isn't much to worry about since the AC isn't electronically controlled through the computer. Well other than hanging your R4 compressor on the front of the motor. Pretty sure you can buy brackets for that or I've seen brackets from an S-10 modified to work.
I threw a 5.3 in my 88 Jimmy, it was pretty easy and it is awesome in the mud.