Dude told me my 5.3 was out of a 2001 z-71, but who really knows when you are talking salvage yard motors. The place is a chain called LQK auto parts and they sell the motors for $600 up depending on mileage. Chevy put them in nearly every truck/SUV application and some cars and SSR's have the all aluminum version rated at something like 325 hp stock. LQK had 20 something of the engines in stock at the time and that is why they are so cheap. There is some expense involved just like any engine swap, but once it is in there, I can use and abuse it knowing that $600 and an afternoon will get me another engine. Hardly worth even considering building one these days.
I did do one extra mod that is not entirely necessary but is nice. That was replacing the stock 6 quart pan with a 5 quart from an LS1 Camaro. It cost about $250 but shortened the pan 3-3/4" making it level with the TH400 pan. I worry alot less about damage than I would have with the 6 quart.
The harness, did not come with the engine, but a friend hooked me up with a factory harness. It looked like a bird nest and even had the taillight bulbs still attached. I sent it off with to Wait4MePerformance in Indiana and he modified the harness for $325 and took out all the unnecessary stuff and cut it to length. I also bought a computer from him since I didn't get it from the junkyard. The computer was $100 with programming included. I don't know the particulars of the programming, but told him it was going in a rockcrawler so low end power was desired. He tuned it to make about 315hp (up from 285 stock), took out the electronic transmission controls, speed sensor circuit and Vehicle Anti Theft System. It comes back as a 5 wire hookup and it's ready to fire.
There are some other little tricks to installing the heater and coolant hoses and temperature gauge, but the rest was a breeze..especially if you have a little fab experience for the mounts and such.
I was running an AMC version TH400 with Novak adapter to my Atlas. I acquired a Chevy Th400 housing and had my internals installed with some new packs for good measure. It's been in there for over 2 years now and beat hard with no overheating, no loss of oil pressure or anything. It's tight and makes me grin every time it fires.
Several buddies run LS1 motors which are similar platforms. The 5.3 won't hang in a drag race, but it holds it own on the trails. I can also run all weekend on a 15 gallon fuel cell and still have a few gallons left.