It sounds to me like the car is a condition 5 in 'Old Car price Guide' group of condition categories. They only provide values for Valiants in conditions 1 - 4. In condition 4, they show a 4-door sedan value of $1,600. If they listed condition 5, it would probably be around $1,000 or less. You can also look on the following links (be sure you don't over-rate the car!):
www.nadaguides.com www.hemmings.com
www.manheimgold.com