Think about it. In case of a roll over in a 65 convertible, which part of your body is above the trunk level? That's right, your head. If you stay in the car, you will be decapitated in case of a roll over. Now, since a small child's head is below the trunk level, they could survive a roll over if they are properly strapped in. That calls for a three point hookup. Now, start researching how to install a three point hookup seat belt in the rear seat, and get a roll bar installed.