wheres my oil preasure going

Just purchased a 360 dart sport 4 speed 10 days later spun rod end result block bored cam bearings sized rods differant crank new high volume oil pumpreinstalled all run good but after oil gets hot preasure drops at 900rpm to about 10lbs from 45 when cold runs about 37 -40 at higher rpm I came across in book an oil passage plug in block under last main cap pn3462871and wonder if it was installed , when i got the car the preasure was the same also and the old owner said it was like that after he built the first engine does anyone out there know if this plug is not installed will it bleed off preasure thanks neeed help