It's like Chevy knew one day I would want to drill a 1.25" hole in the firewall and therefore gave me this.
a 1.5" diameter formed hole from factory with a pre-punched center. This is all part of the massive wiring job I am slowly trying to get around to.
I had picked up the Daystar Firewall boot.
I grabbed my Stepping drill bit that goes just upto 1.25". I first made a very small pilot hole and went back underneath to make sure that was the correct spot. It was. Aside from some insulation on the inside I didn't have to move anything out of the way.
The crescent shape is visible on the inside and the arrow points to the punched centered hole. This shot is near vertical.
Drilled from the outside and cleaned up the edges. I took some 100% clear silicone and applied it to the seal area where the body panel goes. to make sure it seals good. I then pulled it thru from inside to out. Done!