At Shelby workers needed to make a hole in the firewall to run the oil pressure line and some wires through. It was typically located towards the upper edge, in the middle but towards driver's side.
Ran across this picture of a car I was recently looking at (in the 900's) where I snapped a picture under the dash just randomly. To my surprise it turned out to be a decent picture documenting the process used to make that hole.
You can see the center of the hole still hanging from the opening, some of the firewall padding still clinging to one of the sharp edges.
Just thought I would share
