oil pressure, I'm still buffaloed

Where is this #5 plug you're talking about. Do you mean the rear main cap/oil seal area? I assume you need to drop the pan to see it. I have a 72 shop manual but I'll be jiggered if I can find a diagram where all the internal oil galley plugs are. To answer your question an "experienced" machine shop di the engine prep. and replaced cam bearings, I built the engine but did not focus on the galley plugs (assuming the machinist put them in)