1---Just because an engine is externally balanced does not mean that it's the same as a different type engine that's externally balanced, AKA, 360LA and 360 Magnums are both DIFFERENTLY externally balanced just like 340s and 360s are different, etc.
2--This means that unless you can justify the part # on the aftermarket flex, it may not balance the complete setup
3--If the internals of the engine have been changed, IE the balancer, the torque converter, the rods, the pistons, ANY of the rotating mass, then it won't be in overall balance anyhow.
The 70 440 sixpacks are a good example. So far as I know the only thing the factory changed between the 69 neutral 440s and the 70 engines is larger, heavier rods, and presto, "external balance."
