installing new wiring harness thru firewall, where to drill holes for passthru? Options

[mightyohm]

I am working on a Megasquirt conversion and I need to pass a 1" diameter harness and a smaller vacuum line into the passenger compartment to my ECU box which will sit in between the seats.

I am looking for advice on where to punch thru the firewall. I am hoping to keep the stock backpad in place with as little modification as possible.

One question I have, is where is the firewall a sandwich of 2 layers of metal, and where is it single layer? Does it roughly correspond with the crease just over 3/4 of the way down? I would like to avoid this area if possible.

My current plan is to drill a hole in the center between the seats, a few inches above the sharp ridge shown in the picture, and then run the cable down under the backpad to my ECU box. That should allow the harness to come in from the engine compartment without having to go down through the engine tin.

Comments or suggestions?

Can anyone draw a line where the sheet metal goes from 1 layer to 2 on the photo?