Tell him to shut up. If he's not looking to buy it, his opinion doesn't matter as to what it's worth. As it's been stated multiple times, it's only worth what someone is willing to pay for it. A Datsun guy is more likely to pay more for it if he knows what he's getting. To the naked eye, it's just a heap.
I remember being approached by someone after buying the car and them trying to make fun of me for owning a $500 Datsun. I recall them mentioning how they paid a few thousand for theirs. My rebuttal; you may have payed more for your car, but mine still has more style,history, respect, better parts, and is more fun to drive then your POS Honda. Plus I get better MPG ;)