Speaking on factory OEM parts here
Given the fact these are production line engine assemblies. Are the cranks and rod/piston assm. made and or machined to the same spec.? Meaning on the engine assm. line say an olds 400 1968-69 were all the rod/pistons the same weight? as any factory rod/piston assm for a 400 motor could go into another motor without a balance problem.
I weighed a few sets(8) of 400 rod/piston assm. and found them all the same weight(Bob weight). Did the same on a few sets of 350 rod/piston assm and found them also to be consistant . This would make sense on a production line.
The factory manuals do not reference a complete rebalance if a rod and or piston needs to be replaced.

To ask the question simpler. Again talking all OEM, no machine work done after the factory. Could you put a complete set of 350 OEM piston/rod assm. onto another factory OEM 350 crank. Without a balance problem?

I also understand factory balance is not perfect either

 

Of course, but I would have the assembly balanced if the car was a keeper..