NO 340 EVER left the factory with forged pistons. Maybe you could get a shortblock from direct connection with them, but there was NEVER any forged pistons put in otherwise. I've torn down hundreds of 340s, all were cast.
tut-tut. you weren't there, man. you don't know, ya know. don't matter how many you done taken apart; **** was different back then, 'specially in Dee-troit.