Your change of rods and pistons does not “require” internal balancing. Internal balance on this would require additional heavy metal added to the counterweights. That can be done if you want, and then you could use a 5.2 Magnum neutral dampener and flexplate, or any other appropriate neutral dampener and flex plate.
As others stated, take your parts to see if it can be external balanced with the correct 5.9 Magnum parts.
I have thought about this recently and I theorize it is likely possible to use LA 360 weighted dampener and LA 360 weighted flex plate and then balance it. I think this is how Mopar did their LA 360 replacement short blocks that used Magnum crank, Magnum rods, and LA 360 replacement pistons. I have verified the same casting number crank and rods were used in Late LA 360s and in Magnum 360s, but pistons were different and balance was different.
Hope I am not rambling on too much again.
