360 oil psi dropped all the sudden

360 very low miles on it like maybe 2000. Changed the cam, heads, oil pan and pickup tube so I could put it in a car. Always carried 55 psi when warm. Made 2 passes and oil psi dropped to 18 at idle and goes to 45 when I rev it. Changed oil and filter. 10w40 with zinc additive mopar filter. Oil psi held at 65 then ran it Down road and back to 20 at idle. Any ideas? No noises from motor