A couple of thoughts here...

Externally balanced means using the 400 damper, which is no lightweight and would not allow the motor to rev as quickly as a smaller 350 neutral balance damper used with internal balancing. Now, I'm thinkin' that the 400 damper was designed by GM to cancel harmonics on the longer stroke 3.75" crank and I don't know if the 350 damper would do the same job over the long run. I doubt it.

With the external balancing, if the motor was ever coupled to a manual trans, the combo would require a special flywheel to replace the 400 flexplate. I know 454 flywheels have been used in this instance, but their eccentric weight is not the same as the 400 flexplate's eccentric weight.

Bottom line, it's cheaper to use the 400 stuff for external balancing, but you have more latitude with choices down the road using internal balancing.