There really are no price guides for Mustangs in "poor" condition like that. The reason for this is that the cost of restoring a car that is totally a mess is more than the value of a restored car.
However, this car will still be worth something to someone, even though they will probably lose money fixing it up. Why? Well, there's the fun of doing it, and also someday maybe these cars will increase even more in value.
Right now, a beautiful '66 convertible might be worth $30,000 to $35,000. So maybe yours could be worth $5,000 to someone? You can always do an eBay auction, put a $50,000 hidden (hidden from all bidders) reserve on it, and see how high it will be bid. The intent is the make sure the auction fails since you are only testing the market. So the bids won't be anything near your reserve, but then what you do is contact the highest bidder (if you liked that bid) and work a deal with him, post-auction.
That's one way to test the market and sell the car for the "real world" price. Naturally you'll have to provide plenty of good photos in the auction.
I can't give you a good estimate on price because I'd need to examine the car's condition, equipment, etc.
But this is the general idea of it.
PS: Actually I thought of ONE price guide that has a "poor" condition category--it's called Old Cars Price Guide. I don't think they are online, you have to go to the newsstand to get one.