1970 340 Balance Problem...Bottom Line

The 360 forged crank is just a rumor. I've worked and researched small blocks since 1973 and never seen any examples or documentation. Just rumors.
I'm sure it's safe to say that they don't exist in the OEM world.

1972 was the transition year for 340 cranks. Some leftover forged units made it through in early '72 model production but they were all cast after that.
Of course, "never say never". Seems Chrysler often had an exception here or there.