When I originally bought my truck as an unfinished project, I was pleased to note that the firewall holes had already been filled by the previous owner. I naively assumed that this would save me lots of work - wrong.
The holes had been filled by taking pieces of scrap metal, smearing bondo on them, sticking them over the holes from the back side of the firewall, then sanding the face smooth. Even the gaping 4-inch heater motor hole was filled in this manner. Just slightly tapping on these with a hammer easily knocked them out.
So now instead of installing my new firewall-mount master cylinder and booster, I get to spend the next few days patching holes
Also please see my thread requesting opinions on how best to fill my firewall, thanks!