1970 340 Balance Problem...Bottom Line

I say that the balancer in the photo does appear to have a counterweight on the backside. But, that is NOT an OEM balancer!

An OEM 340 balancer will have a small counterweight cast into the front side and will be a raised area around the pulley. It will also say "For 340 cast crank only" or something like that. It is obvious.

Since the engine has apparently been messed with, you can't be 100% sure that it's the original forged crank. But the vibration would suggest that this counterweighted balancer/damper was installed on a forged crank.