Ok I think I am losing it. A 9792506 Engine casting number for a 69 GTO is a RA IV block which has 4 bolt mains and a 9790071 coded YZ is a RA III block with 2 bolt mains. I should have noticed how late the casting date was from the get go. A 9790071 is also a 68 HO block when coded YZ. Sorry for the confusion .