This is the tap set that the tap came from. It is the harbor freight set. The 1/4" tap is the same size that you use to tap the oil galley plugs for insertion of threaded plugs.
I used vice grips to grab the tap and pull out the valve.
Here are all three parts; the valve, the spring and the slotted plug that holds the thing in the engine
The results of the test were positive but did not completely cure the low pressure problems. I still get the light but only at idel and only when the oil is up to about 140 temp. At operating temp of 200 at idle, I have about 4 lbs of pressure on the guage. This is about 4 lbs more than was reading before cleaning up the pressure relief valve. My idle is very low. With just a little throttle, bumping the idle up to a normal sounding idle (no tach) the low oil pressure light is now off. At temp and running, I now have about 4 more pounds of pressure. This is a great relief. Now I am up to over 30 lbs at the shift point, cruising at 60 in 4th gear at about 26 lbs.
So I do believe that the gunked and stuck pressure relief valve was a bit stuck open causing lower pressure. I still have a little loose bottom end. Next rebuild I am going with oversized bearings, machined crank shaft and/or all new. Hopefully, this engine will hold these pressure values for a couple more years.