I had a numbers matching '68 350 convertible I sold about 2 years ago. It sounds like mine was in about the same conditions as yours. I sold mine for $5500 and it went way too quickly which tells me my price was quite low. I also sold it in the Northeast and rust free cars are rare up here so that will raise the selling price a bit. I've seen cars go in the condition yours sounds like from anywhere in the $6000 - $9500 range. Check out this site and do a search for Firebirds for sale under collector cars to get an idea.
