A simple rust check on most '60s and '70s cars is to open the trunk. GM A-bodies and most Chrysler products suffered from chronic-leaking-rear-window syndrome that spewed water into the trunk, leading to nasty rust-through. As you can see, this '64 Olds trunk floor is badly corroded. Also look for rusty water stains leading from the corners of the rear window into the trunk, especially on the driver side. This guarantees that the rear window channel will have to be repaired.