Balancing cast crank 340

If it was mine...... I'd replace the cast crank since you are going to rebalance it anyway. You could use an earlier 318 forged crank with your rods and pistons. The B&M flexplate is $80 you should be able to come up with a 318 crank in that neighborhood. Use the balancer that came with the 318 crank and you're good to go with no external balance problems.