In my opinion 20k is too much for something that needs restoring. That's up there with two door values. Some rust repairs and interior repairs sounds like to me it will end up getting completely striped and rebuilt. Probably spend a fair bit of money and time on it. 20K to me should buy a neat roadworthy van that can driven as is no work required.

But if it is all original with original engine (number as well as date code if you have the original paper work) and just the perfect colour and drive line combo that someone is chasing it will be worth what ever they a willing to part with to get it. ie if one GT falcon sold for 1 mill doesn't automatically make the rest worth more it was just the perfect car for the buyer and they wanted more that having the money in the bank!