Mmmmm..... The 360's internal imbalance forces (in the rear half of the engine) are transmitted via the crank bearings to the block, and the imbalance forces from the external weights on TC or flexplate are transmitted to the trannie housing and engine block (via the crank flange/#5 bearing and the pump area). The two equal and opposite imbalances are tied together at the engine/trannie junction to counteract each other.
So actually, running the non-weighted TC or flexplate with a 360 wouldn't cause vibrations at the flexplate-to-crank junction....unless the bellhousing to engine bolts were loose. The external weights (TC or flexplate) are what put any stress on the flextplate.
Of course there are other reason that the flexplate can be cracked....
