I've rebuilt a few 235-261 engines in the last few years.Even with crank clearances as much as .003,my engines have 30 psi at idle ,around 700 rpm, and 40-45 psi at 2000 rpm fully warm after a hiway run with 15-40 oil.Two of these engines are 261's with full flow filters,and a 235 converted to full flow.All have after market filters and plumbing.The last engine 235 I did last year,again with a filter conversion,has lower pressure readings like yours.On this engine,I reused the old oil pump,which I believe is worn.
Anyways,like mentioned above,your engine's oil pressure is adequate for normal driving.If the low pressure bothers you,and you've eliminated the filter as the problem,and the guage is actually accurate ,then it must be the pump or bearing clearances.
Plastigauge is a general guide,but it's not alway as accurate as it depends greatly on user technique.
I hear this low oil pressure situation alot on rebuilt 235-261 engines.I'm thinking again it's bearing clearance.
