When I restored my '36 Chevy PU in the early 1970s I counted over 50 holes in the firewall. They varied in size from sheet metal screw size to big ones left by others who passed heater hoses through the firewall and mounted a brake master cylinder and other objects long removed before I got the '36.
I regard substances like bondo and fiber glass as crutches that cover body damage, not actual repairs of body damage. So I repaired my firewall by filling small holes under about 1/4" with oxy-acetylene welding rod deposits and welding patches into the larger holes, also with oxy-acetylene. The attached photos show what the results look like 45 years later.
Only very simple tools were required, an oxy-acetylene welding torch. a body repair hammer, flat dolly and slapping file, a sanding disc on an angle grinder and some scrap sheet metal. This is why I've suggested to you in the past that welding is an essential skill in vehicle restoration and/or fabrication. After all the non-original holes were repaired and the welds ground smooth the heat-induced warping was corrected by shrinking metal in warped areas to eliminate the local "oil can" swelling of the metal.
