Do you know anybody with one of those flexible bore cameras, or whatever they are called? Might be able to get a view through the drain plug to see if the parting lines in the crank are thin (cast) or wide & flat (forged). Maybe even find a number on it.

It might depend on just how it was balanced if rebuilt as well. Internally with mallory metal, with OEM weight parts or aftermarket, etc. Might need a counterweighted flexplate too if its all not "neutral".