I had the same issue with my ‘59 235 when I first bought my truck in 2019. The engine was rebuilt by a supposedly quality machine shop before I bought it. On cold start, it would peg the needle on my 0-30 lb. gauge. Once warm it would drop to nearly zero. After messing around for over a year trying to determine what the issue was, I pulled the engine, only to find a few of the journals were over ground, necessitating a complete rebuild with larger oversized bearings.
You stated you didn’t check all of the rod bearings and didn’t mention anything about the cam bearings nor the rocker arms. Somewhere in your oil system, something is bigger than it should be.
