got the heater box back in the car and have zero room for the ECM. Right now it's just chilling in the box since I don't have ac installed yet. Is there plug N play extension harnesses to get it into the passenger compartment?

 

As far as I know, no one makes an extension harness for an LS PCM (which PCM are you using, anyway?).

What I'm going to do is mount my GEN III PCM in the factory 3rd Gen location, with the connectors pointing down. It's tight, but it will fit; this is what Pocket did on his swap, and I'm copying a lot of his methods.

The harness will have to be extended, as you know, but here is one option that someone recommended on my thread: Go to the junkyard, get another LS harness, then use that as the extension, working from the PCM plugs back; you'll do half of the soldering, and the wire colors will all match.

That's what I've decided I'm going to do.

 

If using a 4th gen harness, then it'll never fit in the stock location. The best you can do is mount it horizontal above the pass feet covered by the hush pad

There arent and will never be extensions for the LS1 PCM because the PCM side connector doesnt exist