These are the areas I've found to be concerning.
1. Shock towers. Look for cracks, loose or missing screws/hardware, and RUST. Be alert of fresh paint too.
2. Floor panels
3. Cowl area
4. Trunk floor
5. Inner posrtions of the rear Quarters. (accessed through the trunk or from under the car).
6. Front apron panels where the fenders bolt up. Also check toward the cowl and hood hinge mounting areas.
5. Rear window areas from the lower corners toward the trunk, and the C piller areas both sides.
6. Doors in the lower front corner areas.
These are areas where I have found rust on classics. I was fortunate on my 67, but these locations seem to be consistent with water accumulation on other cars I've seen.
Crawl under and check the steering and front suspension pieces for "off-roading" or curb impacts. Another clue would be dented rims. The
tranny crossmenber may also tell you of abuse. These cars are not as forgiving of abusive driving as cars with chassis.
Maybe other members can add to this.