Re: Trying not to panic. Chunk in oil.
 

I agree with the previous statements. Based on the fact you checked and all valve keepwrs are accounted for and that there was only ONE chunk metal you should be OK. I believe it is metal from when the engine rebuilt.

Re: Trying not to panic. Chunk in oil.
 

I'm not going to sit here and tell you it's OK for a shop to let something like that be in a motor after a rebuild because I wouldn't think so but who am I to say. Yea people are human and make mistakes, I make at least a few each day I'm awake.;)
I would be checking everything out good. You can rent a scope cheap enough to take a look up inside the motor. If you don't find anything drive it and have fun. Keep a eye on the oil pressure and go.:chevy: