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GMC Maintenance Manual says this about oil minimum pressure with the engine hot:

Used engine at idle 2 PSI, 2750 RPM 25 psi
New or rebuilt engine at idle 5 PSI, 2750 RPM 40 psi

I wouldn't put up with that low idle pressure. That said, my '52 GMC 228 (same engine design including matching oil pump, main bearings, cam and cam bearings) with 40,000+ miles I have driven it, original pistons and possibly original bottom end runs down the road at 60 MPH (2310 RPM) warmed up and shows around 55 PSI. This reading was taken with a 4" diameter oil pressure gauge that is rated for +- 1%. A hot idle (500 RPM) shows 14 PSI.

I did an autopsy on a 270 that had the problem Chris mentions above where the distributor wasn't sitting in the oil pump far enough. Once the distributor tang had come out of the oil pump's slot it never went back. Totally wrecked the engine.

GMCJammer51 suggested checking the pressure relief valve in the oil pump for debris. Just FYI, the oil pump in the 270 that I did the autopsy on regulated to 57 PSI when cold and being spun by a 1/2" drill motor.

Have you looked at the amount of oil coming out of the valve gear overflow tube (8.4550A below) attached to the rearmost rocker arm shaft stand? It usually has a flow at idle about the rate of an anemic drinking water fountain.