Try: hemmings.com. A lot of car collectors use this for buying and selling. I've used it for years and have gotten a lot of hits every time I put a car up for sale. The magazine has a great reputation as the "go to" source for collectible cars.
Just be sure you are honest in your description. Car collectors have a way of detecting fakes just from the write up on the car. If it's a daily driver and not a trophy winner, put that in your ad. If it needs a lot of work, mention that too. Some collectors are searching for project cars and would be attracted to your ad right away. And of course, if it's pristine, say so and be sure to include a picture or pictures that show it off.
Cars with photos sell much faster than cars with just a write up. Good luck.