If engineers made 200k a year, I'd still be in engineering...
I've got a couple of smallblocks that never had those plugs behind the thrust plate. Not saying that can't be it, but they didn't all come with them. Of course, if I was having this much trouble figuring out an oil pressure issue, I'd probably install them "just to see" as part of the elimination process. It's obviously not one of the usual problems at this point, time to start looking for zebras.