Vibration Problem

If it's been "rigged" in a variety of ways, after so many years, it may of may not have a forged crank in it now. But, yes, the balancer and flex plate are for a cast crank and 340's came with forged cranks through somewhere in the '72 model year.

Might be a cast or forged 318 crank that wasn't rebalanced! I don't have the casting/forged numbers handy.

Make sure the converter doesn't have any balance weights on it. (i.e.: cast crank 340 or 360 application.)

Best way to tell is to drop the oil pan to check out the crank. The link shows a 440 crank, but the visual features to ID if it's cast or forged is the same.

http://www.moparts.org/Tech/Archive/bb/img/crankshaftid.jpg