How much would you pay for this 340?

I have not seen a $500 340 block for a loooong time. Regardless of what everybody thinks the external look is worth, a 340 core is still expensive. Just go look in the mechanical parts for sale. Do I have dollar value? Not without seeing inside, but I bet it ain't $500 for the block and J heads.
i sold a date coded 69 340 short block at spring fling in 23 for $500

forged crank, it was 30 over and needed a sleeve in one cylinder. still had the windage tray. it was gone before lunch. i had sold the x heads at the previous fall fling, they needed a full go thru and one of the rockers stands had pulled the threads. i told the guy $200 and he nearly broke his arm reaching for his wallet. the truck wasn't even unloaded.

but to your point, without seeing inside you can only guess. the market will bear what the market will bare, and some people have to have a 340; and for that, they'll pay.