Looking like a strong possibility that I am going to go with a 5.3 LS engine and 4L60E transmission from a 2004 Silverado 2WD. Assuming I can get the engine, transmission, ECU, transmission controller, and all the related wiring harness everything should talk to each other just fine. The 4L60E has a lower numerical final ratio in overdrive so depending on tire size selected I can probably run the original 4.57 rear axle and have reasonable engine RPM at 55/60 mph.
So as an FYI, there is no transmission controller in that setup. The ECU controls both the engine and the transmission. If you are transplanting the whole thing like that, all you will *need* for tuning is a VATS disable. You might consider also getting the rear O2 sensors turned off at the same time. There are a few extra considerations you might want to be aware of:
A 2004 could have a cable drive throttle or an electric throttle. If it has the electric (Drive by wire or DBW) throttle body, you will need the matching pedal and throttle body controller. The advantage to the DBW setup is that you get cruise control for free. The disadvantage is that you have to deal with the electronic pedal and the controller for it. You can swap from manual to DBW (or vice versa), but that requires a segment swap in the ECU.
A 2004 should have no EGR and no air pump.
A 2004 could have a mechanical fan or an electric fan. The electric fan is controlled by a ground switched relay. If it has the electric fan, you can just mount a mechanical fan on the water pump and ignore the fan output. The ECU is not smart enough to notice the fan missing as long as the engine is not overheating.
The LS expects 2 cats and 4 O2 sensors. The front pair of O2 sensors are used for closed loop control and really should be installed. The cats and rear O2s can be left off, but if they are not disabled in the tune it will set the check engine light.
There is a lot of good information here:
https://www.lt1swap.com/wiringharness.htm about modifying the wiring harness on an LS engine to run standalone. It looks daunting at first, but just take things one step at a time and it is actually pretty simple. You will end up with a bunch of connectors for the engine/trans, 2 connectors for the ECU, a fuel pump connector, +batt power, +ign power, and ground.
