It seems like the 340 cars are definitely getting more than the others. Inalways wonder what a non original one would bring. Not that I am selling mine, but I'm not sure what it should be insured for. It's much easier to find a price for a mostly stock one, but not for modifies ones.
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.