just a thought here but if the front plugs were out oil would flow to the pan but you probably wouldnt have that high oil pressure i agree if rear plugs were out or loose it would be on ground definitely if no external leaks on ground i would tend to believe the oil journal plug under the rear main cap was inadvertently left out not pressurizing the top end of the motor but developing good bottom end pressure reading i wish i had a pic for you but i have seen this happen many times on cars in this area its easy to forget that plug