Just a thought but my brother had the same problem with a 327 that he had. Turns out that the oil pump screen actually fell of of the oil pump itself. Needless to say he thought it was fine at low psi at start up till in the end the cam bearings went out and the rod bearings went out. His engine now sits in my garage with no crank and no hopes of seeing an engine bay anytime soon. So when I built my engine I had my oil screen pick up tube tach welded so as to never worry about that problem.