You already have a harness if it is coming with the motor. If you want drive by wire, get the motor with it along with the harness and TAC.
You just need to move the crossmember back, no need to purchase a new one.
Stock oil pan will work. I am running the stock one on mine and it is drop 2.5/4"
Headers for a C20, El Camino long tubes work, or Hedman husler 3/4 lenght or if you are running a 4L80E, Street and performance
The factory radiator can be made to work also. I modified the heater return for the steam hole on mine.
The painless harness is the complete truck harness, if it is running driving now and nothing is hacked you don't need it.
As for the driveshaft, it depends on what is in the truck now and what transmission you are getting. If it has a 700R4 and you are using a 4L60E it is the same trans, so if you mount the engine in the same spot the driveshaft should work.
You need to think about fuel lines (eliminating all rubber) and fuel fittings for the LS rail and the Sending unit.
You also need to keep in mind how you are going to get signal to the Speedo (as in the speedo needs to be able to read the signal.
Sending unit will be the same. I recommend getting the 1987 version and mount a EP381 pump to it.
If you are buying tanks get the baffled ones. 1987-1991
For the AC, I recommend this, it is what I am using and will be using on my other truck:
http://www.novak-adapt.com/catalog/air_conditioning/g3_relocation_kit.htmRunning the O2 is required, VATS will have to be deleted to get it running, I would delete the rear O2's nothing else needs to be deleted.
Shouldn't cause an issue. Several use them
You can run the computer under the hood. It simplifies a lot of wiring. It is work putting it under the dash, I did it...
The dirveshaft will need to shortened and rebalanced if going from a TH350.
Again, stock pan will work fine.