There aren't any differences visible from the outside. Drop the pan and have a look at the cranks casting number. It's possible that a 400 damper and flexplate were used on a 350. It'd shake like he-double toothpicks when it ran. I mean it would vibrate so bad that you'd know something was very, very wrong. It would take about 5-10 min. of running to kill the bearings and the oil pressure would drop quickly.