Before you do anything drastic, confirm the low pressure with another gauge.

 

Page 6B-3, Fig. 6B-101A--Checking Oil Pressure - Typical V8 Engine of the 1971 CSM demonstrates oil pressure should be:
AT SLOW IDLE - 7 PSI MIN.
1500 - 3000 RPM - 35 PSI MIN.
I've always used the general rule of 10psi/1000rpm - it's general but provides a good ball park jump-off.

It would be better if you provided a little more data:
PSI @ IDLE (note your idle RPM)
PSI @ 1000 RPM
PSI @ 2000 RPM
PSI @ 3000 RPM

IMO, if you're getting 20psi at normal idle, you might have a malfunctioning oil pressure sending unit, malfunctioning oil pressure gauge &/or a possible issue between the oil pressure sending unit and the oil pressure gauge.

 

 

Did you have a functioning oil pressure gauge prior to the engine rebuild?
Was this a new oil pressure gauge installed by yourself or someone else after the rebuild?

 

 

The only mechanical oil pressure gauges I've used in the past 40 years have been hand-held when reading oil pressure.

None-the-less, I suspect the orifice is clogged with debris - it does not take much debris. In consideration you just had your engine rebuilt, the gauge was working before the rebuild I suspect debris has managed to come into contact & lodge within the gauge - somewhere. I don't know (specifically) the type gauge you have, but here's a highly scientific method.

Literally flood it with carburetor cleaner then blast it with compressed air.

 

Sorry, just thought of something. You said you tried another gauge and the other gauge did the same thing, right?
I don't know where you have your mechanical oil pressure gauge installed on your engine system, the type I use I plug directly into the engine oil sending unit orifice/hole. In any case, you might have debris at the point where the line plugs into the engine (or wherever you have your line plugged into the system), or anywhere between where the line plugs in and the orifice where the line plugs into your gauge.

 

 

 

The oil pump getting easier to turn does not sound normal. Vintage Chief's suggestion of putting a mechanical gauge at the sending unit port is the way to go.

If the pressure is low, look at the oil pump and for a galley plug left out. Ask the builder what the main bearing and rod bearing clearances are.

Good luck, I hope this works out to be minor. Don't run it anymore than necessary, only to diagnose the problem.

 

 

check the drive rod from the distributor,might be rounded off a bit

 

 

Strange that it's sitting at 20 psi and not moving. Priming the motor, it should get harder to turn. What pick up and pan? It could be an issue with the pump, something could be blocking the pick up screen partially or the pick up could be too close to the pan bottom. Just strange to sit at 20. Could any of the galley plugs have been left out?

 

The diagnosis that fits the facts as you presented them is what Mark said. Probably the pump relief pressure is set at 20 psi and that is why the pressure does not change with pump speed.

 

Just a weird scenario to relate, I once saw an engine start up after a refresh and the oil pressure was low. But the oil filter was bulged. All of the assembly lube from the engine plugged the oil filter. After the filter was changed everything worked normally. Have you changed the oil and filter after startup?