I agree, it is a shame to cut up an original car like this but when you are building a high end car of this caliber you need to start with the best sheet metal you can find.
I have to disagree with you here. First of all, I am the high bidder on the auction, and if I win, I will store it for "someday" because regardless of how badly you've cut it up, it can and probably will get reversed someday and I will then sell it back to the next owner at my cost. It sounds like a neat project car (like a Matt D. car in fact?) but I'd B*tch slap him if he told me that was his car he was butchering!
There's no need to start with such a nice example to build that type of car.
It sounds like you are building a race car/custom leaving nothing more than the outer shell in original condition? Your shop requires a clean outer shell to do a high quality job with all of the other custom fab work you do? If so why? Seems like all you needed was a decent roller junk heep without too much quarter panel rust and you'd be good to go? The amount of value damage you are doing by cutting up that original car FAR outwieghs any monitary difference in extra hours the owner would have to compensate you for in terms of repairing a car in lesser condition.
Sorry, I'm passionate about these things, don't take it personally, it's my hobby and it just kills me to see that happen to such a nice example. Man what a pitty.
*Did you save the good metal you cut out of it? I'll buy that as well