This LS swap guide is from the perspective of a 2003 5.3 and 4l60e out of a Tahoe with the stock wiring harness and computer, but not using any of the stock fuse boxes or relays. The procedure should be similar for all LS swaps.
You are going to need 4 relays (up to 7 if you want an electric fans), 2 fuse blocks, some 12 gauge wire, and a lot of butt connetors.
lets start with the relays,
Relay #1 – Fuel Pump. Please refer to How To: LS Swap Fuel System.
Relay #2 – Starter (might not be needed depending on vehicle)
Relay #3 – Key-On Power (power when key is in RUN position). See key-on fuse block picture below.
Relay #4 – Torque Converter Clutch Disengage. (if using 4l60e or transmission with lockup torque converter)
Relay #5/6/7 – Optional double electric fans. (both fans spin slowly on low speed, and both fans spin faster on high speed)
Now for the fuse blocks, I wanted to use the GM specified fuse values from the original donor vehicle for the proper circuit protection with the small factory wires. I chose to use fuse blocks made for boats that I found on amazon, you will need one for constant battery power, and one for key-on power.
Fuse Block #1 – Constant Power
Fuse Block #2 – Key-On Power