Still need the magnum flexplate?

ANY engine whether a SB mopar, 440, BB or SB chev must be balanced both at the front and rear. IE if neutral balance, so must be both the balancer and "whatever is back there", flywheel, converter, flex, etc.

And likewise weights must be present front and rear, depending on combo. Also bear in mind that things like changing stroke, rod weight or pistons 'by much' will change all this from "original"