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The Owners manual for the 526 states:
"The gauge on the instrument board should show 20-35 pounds pressure while running at twenty-five miles per hour"

1. What rpm is nominal for 25 mph?
2. What pressure should be at 40mph? 45mph?

The engine speed at 25 MPH would be around 1200 RPM depending on the final drive ratio and wheel size, there are plenty of online sites that have the formula.
The oil pressure will not continue to rise in proportion to engine RPM because there is a pressure relief valve on the pump, If it's set to open at say 25 PSI then it will not rise above that at higher RPM.

The old timers always wanted the oil pressure set high on those cars.

I wondered about that and checked for data.

Below are two pictures from the period showing the oil pressure at idle on a low mileage 740. It looks like the old timers weren't just practicing religion.

These pictures show 32 psi with the temperature gauge at the G in range.

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I noticed that my pressure was pegging at 50 on the dash. I was running 30 oil. I then installed a gauge at the back of the engine, thinking the dash gauge might be defective. It was reading 57+-.
I changed the oil to 0W-20 and back off the relief valve a turn and now with the head temp at 160, the engine idles at 18+-, but still goes over 50 at high rpm.
My question is :is this indicative of a blocked passage?
I have a tachometer, so I can test the rpm. That's why I wondered what rpm is indicative of 40 mph.
What's the downside of high psi?

I wouldn't be concerned with road speed oil pressures of 50 or 55 psi. I've set my pressure relief valve on my '34 Eight to open at 55 psi which really only occurs prior to full warm-up. I believe that setting is in agreement with factory advice and I've been driving it that way since the early 1960s. My hot idling pressure is about 15-18 psi, road speed pressures with a hot engine at 45 mph is about 50 psi.

I suspect prolonged excessively high overpressures could conceivably burst pressurized internal oil passages in the aluminum crankcase, you read of this failure on rare occasion. And not applicable to your engine but in the case of 1934 and up senior engines, there are potentially dual risks of (a) bursting the full-flow oil filter cartridge at the seam, (b) causing failure of the heat exchanger core. Years ago I did burst one of Bob Turnquists repro L-6 oil filters. One hears occasional chatter about the possibility of 'hydro-erosion or hydro-blasting" of soft babbit bearing surfaces resulting from prolonger excessive pressure, though perhaps that was more speculation that actual observation, and perhaps unrealistically high pressures.

I was afraid that very high pressures would indicate a clog in a passage which would cause a bearing down line not getting proper lubrication.
What's the chance of that?

Your engine freshly rebuilt or recently disassembled? High pressures could simply be a consequence of close bearing tolerances. Or using too high a viscosity oil for the ambient temperatures and bearing clearances.

If you've got reason to suspect a bearing isn't getting oil, you should be getting symptoms and warnings galore, but maybe the best way to get complete peace of mind would be to drop the pan and pull the caps and take a look.

I believe the engine was disassembled about 6 years ago (before we bought it..). I have drive it about 120 miles in the past 4 years.
Yesterday, I changed the oil to 0w-20 and idled it until the head temp was 160F and the ambient temp was 95F.
It started idling at 39+- and eventually dropped down to 18+-.

Averaging just thirty miles a year? Might be doing more harm than good.

The only internal oil passage in that motor is from the rear main to the rear cam bearing which then feeds the camshaft and the roller cam followers. The 11th series and later motors had the internal passages for filtration.

The only thing excessive oil pressure is going to do here is reduce taxable horsepower. I remember one 6th series standard 8 that went thousands and thousands of miles over the years with the oil pressure gauge pinned. It would go from Miami to New York City and back twice a year. That was during the 55 mph speed limit era. Can't do that today.

David is correct that you should only run the engine if you are going to drive the car somewhere. Garage running is setting up deposit formations, ring starvation, and stuck valves. Fuel stabilizer will prevent storage problems.

My choice would be to set the oil pressure to hit 50 psi at 40-45 mph and don't worry about the rest. It seems to me that motor is in very good condition.

Oh, and I notice in the picture above the throttle is not fully closed. They must have it that way to keep the water circulating in the hot motor.