72 still had forged cranks, the only year they put the cast crank in the 340 is 73...
You can interchange the cranks between the engines as long as you keep the original vibration damper with it so it is properly balanced... You can use the cast crank in an earlier 340 block if you use the proper 73 cast crank balancer for the 340...
As long as you keep the cranks with the proper original dampers, you should be ok...
