Depending on where they are sitting and for how long, they can rust anywhere. I actually have pretty bad rust across the front of my hood! like it had been sitting in weeds for years! I don't have a Cuda I have a Challenger but I'm sure it's pretty much the same.
On mine it's around the REAR window trim, especially the bottom drivers side, and the trunk floor.
Also look around the rear wheelwell openings, the lower rear quarter panels, and the lower panel right in front of the rear tires. These spots are pretty common on theses cars. If any of the windows leaked the floor boards could be rusty too. And ALWAYS get pics of the frame just to be safe.
I would ask the seller for pics of ALL the rusty areas and any areas that show signs of a wreck, or prior body work (bondo etc), and also the specified areas, this way he may show you a few more rusted and/or damaged areas.
Good luck