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)
method two;
1. drill out the tack beads on back of bracket
2. hold in position on firewall
3. weld bracket thru holes to firewall (effectively re-tackwelding bracket to firewall)
method three;
1. who needs a gas pedal? just tie some bailing wire to the cable end to extend it, and reach down.
2. pull on it when you wanna go.