I would suggest finding a professional appraiser. Sometimes an insurance company that specializes in antique vehicles can recommend someone. As someone who lives in an area where rust problems are common its important to get a thorough inspection as an external visual is not always accurate. Both cars are desirabe collector models so even though not in the best of condition will still have a value much greater than scrap. If you do decide to sell them I think this Chevy site would be an excellent place to advertise them.
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steve6