I can't believe that the skirt on the piston is broke when it was never really ran.
Are you sure the correct pistons were installed in that engine? Seems like the skirt may have been smacking the crank on the down stroke to do that kind of damage.
That was also my thought Mad... I fixed a 416 done by a local shop that has two pistons in backwards. It seemed fine during the break in, but hit valve and piston when driven hard.