351 Cleveland 2 piece valves

Go here.

http://www.network54.com/Forum/119419/

There are several members who had this happen. It can happen even with stock cam/springs. You definitely don't want to put heavy springs and a high lift cam with stock two-piece valves.

http://www.network54.com/Forum/119419/thread/1181086565/Final+Detail

There's a recent thread on this topic complete with scary photos.

Seen it happen. Was out with a friend who had a 71 Torino GT with a 351C. It wasn't stock but it wasn't radically built either. We were out playing and it started running rough. We headed back to where he stored the car. He shut it off when we got there. Got out to open the barn doors, got back in and it wouldn't start back up. We pushed it in the barn, popped the hood and pulled the valve covers. Found the #1 intake valve stem sticking down in the guide further than normal and missing the keepers. Pulled the head and found the rest of the valve in cyl # 4. We figured the keepers popped off. Needless to say it beat up the head, 2 pistons, 2 cylinders. I don't know what he did with that motor as I was on leave from the military at the time and had to go back but he did build a more radical motor after that for that car. Its just extra insurance to get the multi groved aftermarket valves & keepers just like doing the oil system mods.