Just a little donation to the knowledge pool. I found on my teardown and re-assemble of this GM Rebuilt Crate Engine that the timing mark tab is adjustable and it was off by about 5 to 6 degrees. So, I made sure I had #1 at exactly TDC with the heads off so no guess work. I adjusted the tab and bolted it back down so that the timing mark is exactly on 0 degrees. That explains why the timing never seemed to be set correctly...