Sounds like over revved with weight. I was always told that if the rod fails on the big end but away from the bearing its usually from a flexing rod. That happens with high RPM or an imbalance, and those pistons are forged heavy may as well be made of lead. Super strong but heavy. Does the piston travel in the bore , can you knock it up through the deck? Any evidence of piston seizure. What kind of distortion does the broken ends of the rod show. Got any pics?