While we're on the subject of welding, I had also been doing some research on Vintage Air A/C systems and found out that I wouldn't need those gaping holes in my passenger side firewall anymore. Time for some home-fab patch panels.
Gaping holes:
I took plain printer paper and tapped it with my hand around the existing ridge at the back of the holes to create accurate templates. Paper cutout and freshly sliced DeVille fender patch:
Handy magnet:
Aaaah, I do so love the welding blankets I bought years ago, well, except for the fact that this one is made of itchy fiberglass:
I should have slowed down a bit after that, but I did manage to later beat back most of the heat warp I created on one side. The next hole over was going to be a little trickier because I'd run the engine computer's wiring harness through it. In the '88 C3500 there were a few relays that sat on the engine bay side of the firewall alongside a power distribution block and were all covered with a plastic panel while another large loom ran inside the cab and to the engine computer which sat above the blower motor. I really wanted to smooth out the firewall, move the relays inside the cab and basically have one hole through the firewall for all of the wiring to pass through. One problem was that the engine wiring harness ran through a big goofy rectangular glob of resin which sealed tight on an equally big rectangular hole in the firewall. I trimmed that down as much of the resin as I could but it still wouldn't fit through a reasonably sized round hole where I could mount a regular rubber grommet. My solution to this particular problem was to simply cut a chunk of the firewall out, squeeze the engine wiring loom to an existing hole and then weld everything back up. It sounds a little convoluted, but I already had all of my metal working tools out anyway.
Yes, I really went through all of that work to move the wiring harness over a few more inches. Here's the 'smoothed' firewall before any real finish work took place.
I still had to fill in the old blower motor hole anyway. Plus, as I did more research, I held off smoothing and painting this portion until the A/C unit was placed and braced because more welding would inevitably be involved.