1971 340 L2316 pistons stock, modified or damaged?

Houston we have problem here!
At some point in time valves made contact with the pistons, their are now junk.
That hard of contact can cause problems in the rotating assembly in particular with the rods.
Replace the pistons and have a reputably engine builder inspect the rest of parts before reusing any of them. My first guess would be that someone put in a cam with to much lift and/or the wrong ratio rocker arms in it. Or the pistons were installed incorrectly.
Good luck
Edit: I now see that others members have agreed that the pistons were installed incorrectly.