Answer: If the car is totally original, and not restored to original, then I wouldn’t try to clean up the light rust on the frame or in the engine compartment. If the rest of the car is in excellent original condition, and the rust is old, superficial, and limited to areas like the frame, we give it a different name. It’s called “patina.” Patina is a much fancier word than rust, so we can charge extra for patina whereas we have to discount for abrasive words like rust.
The 1966 Impala SS is a very desirable car amongst collectors, particularly with the big-block 396. It’s not a particularly difficult car to sell, but the amount of money that the sale will generate will be directly proportional to how much effort you’re willing to invest in marketing the car. An original, low mileage car such as this will generally do well at one of the major auctions, particularly if there is substantial documentation. This option is expensive and labor intensive, so you could opt to advertise it online on ebay, Hemmings, or any other website, but you will likely realize a lower sale price.
The value cited by your insurance company is likely accurate, although I would put its value at the lower end of the scale unless it is a 4-speed with some original documentation. Some extra options wouldn’t hurt either.