"rocks in a blender" sound in my wife's slant Valiant

Maybe put in an after-market one built for racing (solid disk). The way it broke, it appears like brittle steel, not tough steel like a flexplate needs, but perhaps there is an axial mis-alignment between the engine block and tranny body that made it flex every rotation and thus fail from fatigue. I found the flexplate in our 3.8L Mopar engine cracked all around the bolt circle, and shifted 20 deg from the crank. That seems to be a common failure, probably due to poor steel.