As a rough ballpark, you can try www.nadaguides.com and click on "classics". Keep in mind that HIGH retail value would mean a car in impeccable condition. If your car has some little dents, or rusty wheels, or tears in the upholstery or a dirty engine, it's not going to bring High Retail by a long shot.
The other advice posted here by joseph is good advice---just put it on eBay with lots of photos and place a very high reserve (secret) price---one that would be impossible to meet, or a price that you'd be happy to sell the car for. Don't reveal this reserve price to anyone who asks---let the auction play out and once it fails (because reserve wasn't met) you at least know two things: 1) you have a list of people interested in the car and 2) you know what much of America is willing to pay for it. The more details and photos, the more accurate your "appraisal" on eBay will be.