I posted this on thirdgen.org and figured I'd copy it over to here since this is where I get most of my LT1 related info and I want to give back a little. So here goes:

I've read some threads on this and never saw definitive pictures of it being done. Many said it wouldn't reach, but I decided to try it out myself and made it work. Here are the step my step pictures:

I decided to use an existing hole in the firewall and enlarge it to run the ECM harness through. The hole is just below the non-ac heater box that I have. I'm not sure if it was originally used for my a/c heater box or if it was for some other option that my vehicle didn't have


from the dash side:


I used a 1 3/4" hole saw. I had trouble keeping it lined up because I had the auger inside the existing hole. I went slowly and it didn't take long to break though.


engine side (after cleaning up the cut with a drum sander):


I found that it was easier to feed the harness through if i separated the top plastic piece from each of the four connectors.


here it is passed through on the dash side:


I couldn't find a large enough grommet in any stores, so I ended up using the stock LT1 grommet that was used for the C210,C220, and c230 connectors. I cut it open so I had a shot in hell of getting the larger ECM connectors through it:


It's good this grommet can stretch because it took some work to roll it over each connector:


Here it is all put together with the ECM attached:


And the whole reason for this work... a nice clean engine bay where the wires will be tucked under the heater box:


What's left to do is to mount the ECM in place behind the dash. I'm expecting to fabricate a mount for it that attaches to the black plate behind the fan motor. I won't do that until I get the dash back into the car though. I had my dash out for other reasons, and I would expect that this work can be done with all the dash and HVAC stuff installed. I'd suggest that someone try using a 1 1/2" hole saw because the grommet is a little loose in the 1 3/4" hole. I'm going to put one or two layers of tape around the inside diameter of the hole to get a tighter fit on the grommet.

So I'm just here to share with everyone that it CAN be done and looks very clean! I see no reason why this wouldn't work for an LS1 swap also.