'65 Cuda Fastback
The rust areas you want to check are up underneath where the torsion bars go into the crossmember, just under the front floor pans. That's a common area to rust out. Also check right up underneath the front, behind the wheels/shock...right in behind the shock and upper control arm. Mud and dirt packs in there and the frame will rust there too. Usually the front floor pans are rusty too, most often from wet carpet. Why wet carpet? Because the front cowl/air plenum area is usually rusty...check up inside the drivers side air vent. Reach right up and feel the metal up there. Lift the rubber around the lower part corners of the windshield if you can and look for rust in there too.
Rust around the gas cap isn't really a problem if it's just surface rust. If it's rusted right through it's a lot easier to fix than the areas I've described too.
Don't get caught with problems like this Barracuda here has!