I've seen this many times. Usually a result of piston to valve contact. Also seen it from the retainers not being seated quite right. If a rocker breaks it will bend and break the valve stem sometimes. Pm me and we can discuss further.
There are many things that can cause this. I doubt faulty valve is the issue. Post pics of the piston. Did you install stiffer valve springs? Reuse the retainers?
Wish I was partying. With being away for work two weeks at a time and having two kids and my wife keeping me busy I have limited time. Oh and I build turbo kits on the side. One say I'll put my truck back together.
The flex plate will bolt up and the engine will run. The engine balance will be off. Best to put a 6.0 flex plate in and use a 6.0 convertor. There are some flexplates and torques that do both applications.
Use a 2200 stall converter and get some better tuning. Upgrade all the shafts. The 472 is a little bug for stock fuel but with a looser converter it'll work.