I think if you are letting one go with a 340 in it that should be in the price. if the buyer isn't ponying up the cabbage, yank the engine and drop a 318 or 360 in it. This is on a non original car and IMO.
I don't plan on selling it, but mine has a 440 in it. You don't see too many for sale with big blocks that aren't drag cars, and built as such. Mine is a 440, 833, minitubbed, with QA1 suspension, 4 wheel disc breaks, a full interior, console... Kind of a mis-match, nothing is original, maybe the door panels, and the dash, which has been cut for a modern radio. I assume it's in the 22-26k range.