Don't want too get in a pissin contest, but there's not way in hell you can tell if it's a 406" by the balancer in a built motor. Every crank manufacture sells the crank with neuteral balance or external balance cranks. I've got 2 406" motors and a 434" motor, i can assure they all run 350 balancers (neuteral balance) you have too look at the block too know the difference. Even the cheap rotating assy's come either neuteral balance or external balance, your choice.
Zip.