A few diagrams to look at for the LS3
https://cimg9.ibsrv.net/gimg/www.cor...14bb50de8c.jpg
https://cimg5.ibsrv.net/gimg/www.cor...5bf1923879.jpg

After reading more about the LS3 I`m starting to think you might (maybe) have a cam bearing that has walked out. If there was a bad o ring on the oil pump there would be air in the oil that makes the lifters tap, and you said no tap being heard. I`m starting to think you have an internal leak. My thoughts on this. I could be wrong. taking off the oil pan to verify bad cam bearing. You can run this by your guy and get his thoughts.

Mike: Welcome to the forum.