How far gone is too far gone?

The last H code demon I parted, I have regreted ever since. At the time few panels were available but now it certainly would have been restore worthy. 71 340 4 speed black on black.
If the frame rails are mostly solid I would say its restorable. Of course if its not numbers matching I wouldn't go throwing thousands of dollars in sheet metal at it. H codes are starting to bring really serious money. At the end of the day its up to you to either invest the money in it or part it out to suite your needs. If your goal is preservation you may consider finding someone who is willing to invest the money in it.
Think about the future too. 20 years ago I parted a bunch of cars that are more than fixable now. Back then there was no sheet metal available and little aftermarket parts compared to what is out there now.