Cast cranks are much lighter and need external balancing which can be a pain.
From memory the 318 cast crank was still neutral balance so all good there.
Then the 340 got cast cranks in 72 and the external balance thing raised its ugly head.
Then the 318 got the heavier truck rod too and it gets complex in the 70's.
Its the big-blocks that suffer with the external balancing nightmares...:mob: