Electrical Information for LS Gen 3
The best resource that I have found for LS engine electrical is lt1swap.com. The pinout of all the LS engine electrical is available there. There are also many other useful resources as well.

LS Gen 3 4.8L and 5.3L Types and Where to Find
The engines the ACE method supports are:

LR4: 4.8L Iron block/aluminum heads
LM7: 5.3L Iron block/aluminum heads
LM4: 5.3 Aluminum block/aluminum heads (my choice for weight and price reasons (most junkyards think they are LM7s and price accordingly), will have to change oil pan)
L33: 5.3 Aluminum block/aluminum heads (high compression (almost a mini LS6), higher price, low production volume)
And: *L59: 5.3L Iron block/aluminum heads (flex fuel option -- *I prefer to avoid, more sensors)

Other engines, including 6.0 variants may also work, but they have not been tested.

Anything past 2007 moves from Gen 3 to Gen 4 LS engines. Most post 2007 engines will not run on a red-blue or blue-green ECM without modifications. Gen 4 does not support drive by cable in most cases and is more expensive. It comes with other features to factor in like displacement on demand and variable valve timing, which add to the complexity of the swap and tune. We will move this direction eventually, but right now we’re sticking with Gen 3. For this specific swap, there are not a lot of advantages of moving from Gen 3 to 4+, mostly just more issues to solve.

The easiest way to find engines is Car-Part.com--Used Auto Parts Market. You can plug in your zip code and the year, make, and model of engine and it will find all of the closest options to you. The option it finds includes all compatible models. For example: if I put in 2004 GMC Envoy XL 5.3, it will find 2003 and 2005 compatible engines from Envoys, Trailblazers, and anything else that had that engine. Search terms:

LR4: 1999-2002 Chevy Silverado 1500 4.8 (drive by cable), 2003-2007 classic (drive by wire)
LM7: 1999-2002 Chevy Silverado 1500 5.3 (drive by cable), 2003-2007 classic (drive by wire)
LM4: 2003-2005 GMC Envoy XL 5.3 (drive by wire)
L33: 2005-2007 GMC Sierra 1500 5.3 L33 classic (drive by wire)

The car engines are a whole other ballgame. The accessory drive spacing is different and the cam is not setup for torque. In addition, they are much more expensive. In my opinion, the LS1 would be no better in this swap than a LM4 or L33 with a bigger truck cam from CamMotion or another aftermarket provider.

I’d avoid the flex fuel and the 6.0s and prioritize those with less than 100,000 miles. I prefer the LM4 and L33 because they are aluminum blocks and don’t weigh much more than the outgoing Rover V8.

You can buy a factory rebuilt engine from GM for ~$3700-$4000. You can look at prices here: https://www.genuinegmparts.com/powertrain-parts/engines.

LS Gen 3 ECM Information
The other critically important item is the GM ECM. 1999-2002 had the red-blue 411 PCM, which was drive by cable. This is much easier to install in the Discovery. 2003-2007 used the blue-green PCM, which supported drive by wire. Some of the blue-green PCMs supported drive by cable (but these are rarer and typically only found in 2003-2007 vans and the 2004 GTO). For drive by wire, you also must install the GM gas pedal assembly (not a fun process if you can avoid it) and the GM gas pedal ECU as well as have a different wiring harness and throttle body. Overall, it is easiest to use drive-by-cable setup.