I think if you were going to do a chevelle outline it should be a 70, purely because of how recognizable a 70 chevelle is, and I own a 69 so it certainly isnt based on personal preference.
You could also do a "chevelle" trunk script outline in the background instead of a car, that might be kind of cool.
I dont know that I would do any other type of designator, such as "SS" or " Malibu" or anything like that as you certainly dont want to segregate the site, or make new potential members feel they need to have an SS or whatever.
If a 3 color decal is possible, I think it would look neat to have "Chevrolet Chevelle Forums" across the top, with
www.chevelleforum.org across the bottom all in site colors with a "chevelle" truck script through the middle in chrome.
Just some ideas to toss around
Jeff