1. around rear windshield . Mine was out for a new seal - the old seal had leaked over the years - so check under rear parcel shelf. Rust is developing there.
2. get under the car - have a look around floor pan etc - especially on either outer edge ( roadside).
3. If you have a sunroof - look at the headline for any signs of rust staining - if it is the stains are showing symptoms of something bigger.
Good Luck
I'm sure some degree of rust is common on these cars given age, but when and where should it be a concern.
I do have a body man who is an expert in rust. He spends the majority of his time restoring pre 60's cars. However I'm not looking to do a frame-up.