The externally balanced cast crank 340s still used a neutral balanced flywheel. Only the harmonic balancer has a weight on it. They were able to take care of the balancing with just the weight on the harmonic balancer. The easiest way to check the flywheel is to flip it over. If there are no big lumps of iron anywhere on the back, it will work on a 318 just fine. Only the 360 flywheels have a weight on the flywheel or in the case of an automatic, on the torque converter.