Some firewalls are not completely flat. Some have areas that are recessed where both panels of the firewall are pressed together (no air gap between the 2 panels. On my Jeep Wrangler, I drilled through one of the recessed areas and I had no problem installing the brake cable.
If your firewall does not have a recessed area near the brake pedal, then I suggest you call Ready Brake. I am sure they will be able to help you. They were very helpful to me when I installed my system back in 2007.
Wishing you the best.
Ray