If both engines use a 2 pc rear seal then they can interchange. As stated, once the SBC began using the one pc rear seal, then those crankshafts also require a flywheel/flexplate with a counterweight on them to be balanced, and will not interchange with an engine that is a 2 pc rear seal. The "balanced" terminology from the parts store could be hinting that the app he lists one for is the later counterweighted one.
