I started off by welding up about 2 dozen holes in the firewall. Over the years, previous owners had drilled lots of extra holes to run radio wires and the like. Also, the original firewall sound deadener on the inside of the cab was held to the firewall by push in clips. There were about 12 holes from them. When I go in with the interior, I will glue to new sound deadener in place rather than use push through clips. I like the firewall to be as free of holes as possible to reduce water leaks and engine fumes getting inside.
After doing the welding, I sprayed a heavy coat of filler primer then white paint. The truck isn't going to be white, but back in the day, everyone painted their firewalls white to accent the pretty engine. As done back then, I do now.
I know the pictures are anti-climatic, but trust me, it will absolutely pop when I get the overspray cleaned up and all of the little widgets bolted back onto it.