Wow, let's see...start with a baseline, nice 1964 Impala. 9K
Add:
No Rust. 3K
It's a low mileage car. 2k
Original owner is a grandma, and your grandma to boot (not abused). 2K
It's not a 4 speed, but, see above. No biggie.
It's a Super Sport. 3K-5K
It comes with the original drive train. 3K
Interior is as clean as an operating room. 2K
It has the original paint. 3K
I'm guessing it has all the original paperwork. 1K
And last but certainly not least...it's a convertible, meaning when the top goes down the price goes up. In this case, way up. 6K
I called an old friend who currently owns each 60's year Impala SS built, both hard top and convertible. I told him about your car, and I'm repeating what he told me. Maybe this will help...some.