I would not be interested in buying or holding 40% halves.
Unless the coins are pretty or "collectible" in their own right, I think the primary value is that they could be readily refined into useful industrial or jewelry metal. Silver rounds and bars are already there, and 90% coins can be refined to .999 or upgraded to sterling by adding some .999 But refiners don't seem to like 40% coins at all.
Most people have no idea that 1965-1969 halves have any silver in them, and it would probably be a real hassle to educate them in a barter situation, where 90% coins are obviously different once you compare the edges with a later coin. Even those people who do know the silver content will probably rather not check the dates of each coin (or take a chance with spot checking a sample).
The value of a thing is what you can trade it for later. (Unless you have some personal use for them.) So the question is, "Who wants these coins now, or who will want them in the future?"
In the past, the 40% halves had an interesting angle, that you could always just spend them for face value, and therefore your bag of halves could never be worth less than a thousand dollars. But that was a while ago and I doubt anyone thinks silver will drop to $4.65 bucks an ounce again. (Is that number right? I got it by dividing a thousand dollars by 215 troy ounces per bag.)
Hmmm, I've got close to $500.00 in 40%, ($70.00 of it in BU) bought it when I first got into silver and didn't know better. I'd like to trade it for finer silver but man I would lose a lot just on the premiums selling it and buying pure. I do like the fact that it does have a face value and is a recognizable form of silver.
My opinion is to avoid buying 40% and war nickles at their melt value. It is just a hassle. However in prospecting at banks I love finding the 40% at fifty cents a piece. Itt should be noted that I have no trouble unloading any 40% at a moments notice for a tick over 5X face.