Internally Balanced 360 Cast Crank

My buddy had a stock cast crank rod and pistons, turn it over 6000 all the time. told him he was going to bust that cast crank if he keep turning it that much. We raced every week end in the summer for 3 year before he got married. Still has the duster, still has the shaved 360 head chry purple cam, and stock rotating bottom end.
o and it was in his 68 dart GT, that he race before that.