Yet another question on vibration (Not the good kind)

Yeah, abdywgn got it- the cast crank uses a differently (external) balanced flywheel than the forged (internal/neutral balance) crank.
It's not a matter of "how" it's mounted- they can only go on one way.
If the crank was simply swapped out, then the rotating assembly is out of balance in addition to the flywheel being incorrect for the crank.