Originally Posted by 1FastBrick

I believe the V6 all retained the fuel return line but hopefully he can confirm.

Originally Posted by RedXray

The issue you'll run in to with the 09354896 PCM is it has a Red/Blue connector and the 03+ harness you are looking for will have the Green/Blue connector. The o2 sensors are wired different between the two. The link above has info on how to swap pins around to make it work at the bottom of the page. That 4896 PCM is already programed for that engine so reprogramming shouldn't be necessary.

If the v6 05 (green/blue) PCM has IAC you can use that with no pin modifications but you'll need it flashed to run the V8.

Easiest option 03+ van harness with PCM

The other option will be using an 03+ Silverado, Sierra, Tahoe, Yukon, Escalade (4L60E) harness, PCM (your 05 PCM will work with flash) electronic throttle body, returnless fuel rail (drivers side) and a DBW pedal & TAC module. Check out your local pull-a-part, you may find a deal on either option above.

I'm pretty sure you'll need the returnless fuel rail anyway if your v6 05 only has one (supply) fuel line. The 99 LM7 you have should have two fuel lines a supply and a return.

The intake on the left (99-02) has the supply and return lines whereas the intake on the right (03+) has only the supply line. I have two because like I said above "I feel your pain"

The returnless rail dose NOT have the fuel pressure regulator on the rail as the 03+ fuel pump dose the regulating. (your 99 LM7 should have the vacuum diaphragm fuel pressure regulator on the rail... like the one on the left)

Originally Posted by 1FastBrick

I believe the V6 all retained the fuel return line but hopefully he can confirm.

If it were me, I would use the V6 computer and have it flashed by someone competent. You would likely need to use an 03 operating system because of the return Fuel pressure regulator. Then get any 03-07 classic V8 harness and rewire the throttle body connection for the a manual throttle body. You can likely take them straight from the V6 harness too...

If you know which wire colors correspond to which pins in the DBW Throttle body you can extract them and switch them to the Drive by cable connections. Or you could take the whole harness apart and get the complete wires from the 99-02 harness and put them in the correct spot using an express van Pinout from 03-up

Van harness are longer at the PCM connection they also seem hard to come by. Most idiots in the JY cut them up when stealing parts off the motor in the self help yards.