I just had a experiance similar to Jimmy with my fresh built 406. I had 80-90PSI at start then 55-60 @3500RPM 35-40 at idle when warm. While setting my dist curve I went out on a road test and the
oil pressure dropped to 40@ 3000 and 20 at idle. I found this post by SFfiredog
http://www.erareplicas.com/427man/engine/oilpress.htm and my gasket appeared to have a hole so I installed the fix. Now my
oil pressure has returned to the previous 80-90PSI at start then 65-70 @3500RPM 40-45 at idle. I'm not sure if s/o's have this same by-pass but it's an easy thing to check.
Mike