I seems like you already know that rust is at the top of the list of what to look for.
The most likely areas for rust are:
1. The bottom of the stub frame behind the front wheels, especially on the driver side. Be sure to inspect the inner sides that face the center of the car.
2. Bottoms of the quarter panels behind the rear wheels, inside and out.
3. Quarter panels above the fender skirts.
4. Trunk floor.
5. Look for bubbles under the vinyl top if it has one, and for rust around the rear window.
6. The seam under the hood between the upper and lower firewall/cowl panels. Look closely around the recess where the wiper motor is mounted.
7. All four floor sections. Can look from underneath the car for any perforations.
Beyond rust, previous collision damage would be the next biggest concern.
1. Check the front of the stub frame for crumpled areas or parts that have been cut and welded.
2. Look for cracks or bubbles in the finish, indicating possible plastic filler under the paint.
These cars were not all that straight from the factory, so it's likely that you will see some waves in the panels that have been there since the car was new. Many also left the factory with some terrible gaps between the panels, so bad gaps aren't a sure sign that the car has been in a collision. You'll have to use your best judgement on spotting a car that's been wrecked. Cars this old are very likely to have been in some minor fender benders over the years, and that's not a reason to walk away on an otherwise decent car.
Of course, you'll want to check over the mechanicals the best you can by checking all the fluids and going for a test drive.
Jeff