I hate to burst your bubble, but unless this is a fair weather truck, you may want to consider cutting your repair piece back out.
There is a trough that directs the rain water across the top of that 4" hole and down the side, away from the kick-panel air vent. You can see part of that trough rusted out, in the top of the first picture. Without repairing the trough, the heater fan will pull the water into the heater suitcase and the water may also run into the kick-panel vent.
There are actually so many pieces of tin, spot welded together in there, that it is hard to visualize.
Coincidence that I just cut the inner part of that area out, on mine today. I don't have a rust problem. I'm putting in a non GM AC and I have to reshape the inside of the firewall, as well as eliminate the kick-panel air vent. It's always fun to weld upside down on my back. As I was cutting one piece away, I cut through into the inside bottom of that trough and left a 2" slit that I have to close up. That 4" hole will be closed off. Then a new upper inside corner piece and vent cover welded in.
