I sent this to sgpipes but though I would throw it up here too in case my experience to date helps anyone
To recap my swap is an aluminum 2013 GenIV 5.3L/6L80E combo into a '96 LX450
When I bought my engine/trans combo from pacific fabrication it cam with a new custom made harness. I will post some pics of it. The reason I went with them was that the cost of the engine and harness was really good and the have great reviews and a following on pirate4x4.
I could not be happier with the harness they provided. They use a bussman sealed waterproof fuse box and all top notch metri pack connectors. I new I was mounting the ecm and fuse box under the hood so this was important to me. To make the engine run I only needed to power up one ignition wire from the new harness and connect one other wire to the fuel pump.
They also do just the harness so you can buy from them separately.
One thing to keep in mind, I learned this after and it took some time, was that the 6L80E trans does not have a provision for a neutal safety or back up light switch. If you saw my earlier post, I modified my stock shifter by adding a pair of B&M shifter micro switches for those functions. In order to make everything work I made all my wiring connection behind the glove box where the factory ecm used to live. By using the factory IL1 and IL2 plugs my new engine harness is easy to disconnect for removal if needed. I kept the original toyota engine harness that came out with the engine when those 2 IL1 and IL2 plugs were disconnected. With the original harness I stripped out every thing that was unused and just left the transfer case and speed sensor wiring. I am pleased with the way the electrical turned out. The engine bay is much cleaner and has far less wiring than before. Just a not about the fuel pump and system.
I spent a lot of time pouring over the factory wiring diagrams to see how to power the fuel pump. What i ended up doing was removing the toyota fuel pump resistor that is mounted to the fender where the antennae is. I then cut the plug off and used the wire that goes from there directly to the fuel pump. I attached a single gm weather pack plug on that wire and put the other end of that plug on the fuel pump feed from my new harness ( the new harness has the fuel pump relay and fuse built in so no need for any thing from toyota)
To finish the fuel system I used a corvette fuel filter with built in pressure regulator. This gives you the exaxt 58 psi that the LS engine needs and only runs one fuel line up to the engine, which is all the new Gen IV engines use anyway. Just to clarify, I only used the Toyota fuel pump and sending unit in the tank, in fact I never even took it out, just accessed through the floor plate under back seat. I ran all #6 AN hose and Russell fittings for the fuel lines. I removed all the Toyota lines except for the tank vent line up to the canister. I have not yet figured out how I am going to tie the canister in yet. The filter mounts on the inside the frame to an existing hole just in front of the gas tank.
I hope this helps.