Rod Crank Balance Questions

Yes you need to have your rotating assembly balanced when mis-matching parts like that. If your pistons and rods from the cast crank assembly are being used, they are a different weight than the 6 pack rods and pistons you say were used with the forged crank. The bob weight of the crank is what balances it. Just hope if you get it balanced they don't have to add material as it can get expensive.