Cowl into firewall which as you say kills the floors. Door pillars under the bottom hinge, inside and out. Panel under the rear window (look from inside too). Lead joint rear top corner of doors essentially as the roof gets into the gutter. Sills. If it has rust in these areas it probably has it in others. The chassis brace on these where the front of the tray sits rusts bad where it welds to the chassis but easily fixed if not too bad.

As Beachy says check the chassis for cracks, but that goes for all HQ-HZ. Check at welds where lower control arms rear mount welds to the rails. crack spot is bottom and side welds at the back. Also look for cracks where the "shock tower" welds to the rail.