Learning about the Cast 340

If it's the original crank, it isn't the same as a 318 cast crank. Did the harmonic balancer have a lobe on it, and say "340 cast crank only" on it? If so, that is the real 340 cast crank. The flex plate is the same as any other 340. A torque converter or flywheel for that crank/balancer is different and unique to the 340 cast crank. Not the same as a 360.
The difference in the 340 only cast crank and a 318 cast crank, is that the 318 cranks were internally balanced, where the 340 cranks are externally balanced.