Do you like the car and is the car worth $6000 to you regardless of its actual market price? If not, the current owner should be able to sell the car and then negotiate with you for cash on your truck.
Before trading I would look the car over closely for the correct and matching body and engine numbers. Also look for rust, especially under the floor pans. It could cost $15,000 to fix it up into a $7,500 car. Did those cars have the X-Frames like the full size Buicks?
Can you take the car to an appraiser?
By now there should be a lot of information available on the internet about those cars. In the 1990's MBI (Motor Books International) published some "Illustrated Buyer's Guide"s for a wide variety of car models. They probably published one for Chevy's. The books highlight the options available and they can be an enjoyable read. Classic Motorbooks, Osceola, WI 1-800-826-6600, or E-bay are good sources for those books.