I don't know why they external balance the 489. In my research most of the cast cranks are external balance in 4.25 stroke. I assume it's much easier to just balance the bob weights and use external balanced flywheel/balancer. I am very impressed with the engine as far as performance goes. I caught them during some growing pains and now paying the price for it. The rear main seal also has a small drip. I might as well dig in while I'm there. Now that I know it's two peice I get to pull the oil pan as well.
Frank...you are correct that mcleod makes a separate flywheel for Gen VI 454 and 502. Mcleod said the counter balance weight is the same but the placement is slightly different. I never dreamed I had a gen iv crank in this motor. Thanks for the words about the car.