I don't know of anyone who specifically appraises parts, but I would start with the regular network of appraisers, and if they can't help you, contact dealers of a similar era of GM NOS parts to see if they can help with the values. Both services have a price, but it should be nominal in the overall scheme of things and will probably  end up paying for itself.As a warning, many people will not appraise cars, parts, etc., AFTER they are damaged. However, I have heard some will, but it costs more than a regular appraisal. You'll just have to call around to some appraisers and find out. (For a list of appraisers, look on www.oldcarsweekly.com or see the pages of Old Cars Weekly).