How much?

If you do a quick search through the for sale forum (try searching with "slant six" and may want to try other terms as well), I think you'll find that most have tried to sell them for much less than $500 unsuccessfully, even with trans. Sometimes successfully, but their starting prices were much lower unless it was rebuilt. Even Super's. But you might get lucky and find someone right in your area looking for one and get more than the going rate, who knows (still, $500 seems unlikely to me). I'm kind of interested in getting a complete /6, but wouldn't be interested in it for anywhere near $500, but that's just me, so local is my only option as shipping would be too much from anywhere.

You also didn't really give enough info. Is this rebuilt, how many miles are on it, etc.

I think it's so hard to put a solid price on something like this that doesn't have much demand for it. Good luck with the sale though.