I think they are valuable, especially for someone looking to restore back to original.
The tough thing about swapmeets is the fact that you basically have to take the sellers word for the condition of what he is selling.
It's hard to check out an engine block, unless you have a portable magnaflux kit, which I believe they do make nowadays. I thought I read a similar post about this subject awhile back, only about cylinder heads, which is basically the same situation.
You'd probably have better luck selling it with a newspaper ad, at least then, whoever you sell it to knows where you live, which would probably give the buyer a little piece of mind. But to answer your question, yes, they are worth something, theres only so many of those blocks left around, and I'm sure somebody could use a good one. KZ