I put one in a corvette cab.
My approach there was to take a 3" piece of 2" angle and clamp it to the firewall, then clamp a piece of 3/16 plate to the side perpendicular to the firewall, then clamp the pedal to that.
I fiddled with it and adjusted all the clamps until I was happy with the pedal location, then welded the plate to the angle and drilled holes to mount the pedal to the plate.
I trimmed it all down to look nice, then realized that I was flexing the firewall from how much it was levering on the mount.
My fix for that was to weld a 6"x6"x3/16" plate to the bottom of the angle and bolt that down. Probably overkill, but it got the job done.
Is that the original bracket for the pedal? If not, you might want to add a small gusset from it to the platform to keep it from flexing left to right.
