Here's my wag not to be confused with reality. I have owned 3 '69 L78 'velles and sold one of them (Colussy, Chev in Bridgeville, PA - Framingham built).
Regarding the car you mention, it is hard to tell without seeing it or pix. Altho green/green cars were quite popular then there are fewer takers now, so that affects market value. Documentation will be real important (don't forget old pix you may have). Sounds like the car is orginal but tired possibly w/o restorable original paint. Undercarriage unknown. With low miles it could be o.k. as long as it missed those messy/salty western PA winters. Storage of the car would be critical to condition. My estimate with info provided: With good docs - $40-45K, w/o docs - $25-30K. Sentimental value to you (original owner) .... priceless.
Am sure some others with more experience in this area will chime in. Keep us updated and good luck - am assuming you may make a play for this car.
