Learning about the Cast 340

Purchased a cast crank 340 engine over the weekend. The price was good and always wanted a 340 :). I'm not too familiar with the later cast version though. After taking the engine apart last night the cast crank is the same one found in a 318.

So did mopar install the same 318 cast cranks in these 340 engines then added the weighted dampner and special flexplate to account for the heavier 340 piston? Is that it or am I missing something?