my 340..3 cars, 34 years, 1 family. where to go from here

Stock rated, yes, but I believe the stock pistons were forged (and very heavy). The ones pictured are castings with the captive ballast weights...and that 10.5:1 number was just a factory optimistic rating. Unless the decks have been cut, it's probably closer to 9.5:1...which is still plenty compression for that crane 284 cam.