I was going to go with JB-Qwik as suggested by trailbeast, but then I realized it meant another trip to the store and I just didn't have it in me. LoL I went with what I've got on hand, QwikSteel. I used some painters tape from the outside to trace the openings and give myself a guide. Then I tore out the cracked pieces and molded in the qwiksteel. Everything looked good and I got it sanded alright. I thought I would work on putting some other things together while it finished curing.
Well, having 6 kids can make it hard to focus and the short of it is that I put everything back together without putting the foam gasket and plate over the pipes for the heater core. At that point my options were to either leave it off or take everything back apart just to get the stupid plate slid over the pipes and screwed into the box. I chose the former. You can judge me for doing this half-assed. That's okay. Everything is back in the car and I will be warm driving to work this week. When I eventually get around to treating the floors/firewall for rust I will put it back together the right way. I was a little worried when I turned the blower on and heard a terrible squeal. Luckily it turned out to be air-flow related and a couple of adjustments to the flapper control lines cleared it up.
Quick PSA: If you find yourself using QwikSteel epoxy putty don't use thin ntirile gloves. I usually use some heavier disposable gloves from the auto store, but I just grabbed what I had in the pantry. The thin nitrile gloves reacted with the qwiksteel. I could feel the heat from the chemical reaction and when I took the gloves off there was qwiksteel epoxy on my fingers even though there were no visible holes in the gloves.
