WF:
That's GM PN 12558060, as below.
http://www.sdparts.com/product/12558...Assembled.aspx
I am not trying to get you to chase a unicorn but I am trying to figure out if your heads were modified enough to handle a higher lift cam. In my uneducated opinion it looks like you are running too much lift for stock Vortecs with a spring change, and it sounds like that's what you have. Post a close up pic of the top of your cylinder heads/valve springs if you can.