I would :
Method one;
1. grind the tack beads off the backside of the bracket so it will set flush onto the firewall, if it will not at this point.
2. drill at least 2 holes, one toward the top of the bracket, and one toward the bottom, being sure they do not conflict with anything else. (Be sure nothing interfears with pedal travel)
3. use bracket held in place at firewall to mark corisponding holes, and drill into firewall.
4. use smallish bolts and nuts (1/8"?) to secure bracket in place. (use locktite on bolt/nuts)