After many years of dinking around with this stuff and wondering: 'why did the factory do such a lousy job on the oil holes to the mains that come from the oil galley? They have "steps" them that hurt flow" etc

Well I think I know why they did that, now.

On some blocks, despite appearing big and beefy, the casting isn't especially thick in the area the oil hole needs to be. If you add in just a little core shift in the wrong direction, then try to create a little too large of an oil hole....the SIDE of the oil hole can break through the casting into the crankcase, and you've got next to nothing for oil pressure.

I've recently encountered this exact screw up... the previous owner gave up on the engine because of oil pressure problems. I got the block. Autopsy shows the oil holes to the mains were sloppily drilled to 5/16" + some drill wander slop. And I found a pinhole in the exact location mentioned.

I am pretty sure I can repair it with a tube, which is a puzzle I'd rather not work on, but will save the block as long as the repair is done correctly.

On his site Dave Hughes recommends 1/4", Chuck Senatore recommends marginally larger 17/64" done with a reamer to avoid "wander". So far I haven't seen anyone wholeheartedly recommend 5/16 but it appears someone thinks it is a good idea??

Beware.....Going too far with an idea or executing it the wrong way might create an expensive repair at best, scrap at worst.