416 low oil pressure at idle

Only solution is to teardown the engine down. 10 psi for every 1000 rpm is Chevy talk. Even my race motors have had 30 psi.at idle. 70-75 from just off idle to wot. It's a clearance issue not a plug. If a plug ismissing. ..no pressure to speak of at any rpm. It'll be easy to figure out when you tear it down. You'll do more damage running it trying to figure it out. I had too much main clearance on a 360. Needed to line honed. Same issue you're having. Kept messing with it till I ruined the crank. Proper machining and it's 9 years on that build.