How much would you pay for this 340?

You have no idea if it's standard bore or a worn out .060. That makes a huge difference in value. J heads are every bit as good as X heads with one possible HUGE advantage. X heads are known to crack between the intake seat and spark plug hole. I haven't heard of that with the later heads. With that said I've got two pairs of X heads, but they've been magged and are good to go.