340 heads??
Not as much as you may think.
I'd say 200-250 max if std 340 heads. X heads get more money than O or J heads. The valves have value being new. The heads aren't all that rare and the same as J heads. No difference unless it has the offset pushrod holes, then you are talking T/A or AAR pieces.
Otherwise they are just 340 heads.
I've picked up complete pairs O, J and sometimes X heads for $50-200. They all needed to be refurbed.