Here's something that happened to me after I had my engine rebuilt by and experienced builder: our engines have alignment pins on the crank shaft to correctly align the flywheel on the crank. The holes those pins go into go all the way through the crank flange into the inside of the engine. I had two pins missing and I was losing oil on the garage floor bad. I was convinced it was the rear main. I changed the rear main out with the engine in the truck. It did no good until an older gentleman mechanic told me what it was. I dropped the pan, again, got some pins, pounded them in and that was it.
Check for those pins.
