One more piece of the puzzle wrapped up tonight. Although we never found the complete accelerator pedal at my dad's, we did find the mounting base which is all I needed for the time being. I took a piece of 1/4" flat steel I had and cut it down to be just slightly smaller than the pedal base itself. Then I drilled the three holes for the mounting studs. After I had that figured out, I drilled about 3/4 of the depth of the plate with progressively larger drill bits so that I had a place for the weld to sit. Then I cut off the heads from some bolts for the studs which I tac welded into the holes. I did not fully weld at this point because i did not want the heat from my welds to pull the studs out of alignment. So I put the studs into the original captive nuts in the firewall and then welded it home. From there it was just some grinding to remove the captive nuts and spot welding the plate into position using the original holes!
Sorry for the long winded explanation but I forgot to take enough pictures to show how I actually did it. Here are the few I did take.