[c6ae]

It sure sounds like you have a clearance issue. At this point I would at least drop the pan and use some plastigage to rule this out.
Calipers are not accurate enough to measure a crankshaft for clearances.
In the standard sizes, bearings were/are available in.001 and .002 undersize to take up for normal wear or initial clearance deviations and the marking of these may be not be immediately apparent. Is it possible that your engine had these bearings?
There is also a 427 specific rod bearing that has more clearance built in (.0005 thinner shell) but I don’t think these are still available.
You shouldn’t rule out the relief valve located in the pump, it might have some debris lodged causing the relief valve to stay open. This would give similar readings on the gauge to what you describe.