Stopped by the bodyshop today and they'd already cut out both cab corners and fabricated replacement metal for repair of the bottom of the kick panel. Turns out when the original owner repaired rot he cut out the outer corners but worded flat metal over the inner. It shows surface rust, borderline crusty but not rust-through. He will grind away these old patches and depending on what he sees will grind smooth and epoxy prime or cut out and replace.
I discussed using a Fiberglas cover (ugh) over the hole-y firewall to save the hours of bodywork and the cost. He said he'd prefer if I let him do the body work on the metal and thought he could do it for about the same amount so I'm happy with that. I'd much rather have the original embossed firewall panels than a fully flat firewall.