Yes you haven't given us much to work with. Would guess its missing more than you listed (smog, exhaust there?) And I don't think there are really less everyday since few are being driving and wrecked as they once were. You didn't provide any information about the condition of the body, rust repair, condition of the floor.....
Trying to read between the lines but it seems the question is, based on how you presented it in your first post, is the car worth what they are asking. Sure you have taken what they think its worth and compared it to the current crop of 70 Boss's out there for sale over the last 6 months. Always allot of 70's for sale it seems unfortunately sellers often take/remember the asking price and we don't often know the selling price
but the best advice I can give you is comparative worth. See what is out there, compare those cars that are similar (you will never find an exact match) and think to your self, would you rather have the car that is being advertised than the one your looking at. Don't expect to flip the car and make any money and over time expect that you will loose money or at best break even.
If you still interested for negotiation figure $20K for a repaint (just ball park) and $500 - $3000 for each part that its missing that is specific to being a Boss 302. Formula isn't perfect but neither is the amount of information you have presented to help (said in a nice non-negative way - just the facts
And yes you should or could ask over on the Boss 302 Registry forum but think you get a similar offer. There are many threads over there on the site with similar questions as yours. Good luck with your choice